HANDICRAFT  SERIES. 

A Series  of  Practical  Manuals. 

Edited  by  PAUL  N.  HASLUCK,  Editor  of  “Work." 

Price  50cts.  each,  post  paid. 

House  Decoration.  Comprising  Whitewashing,  Paperhanging, 
Painting,  etc.  With  79  Ehigravings  and  Diagrams. 

Contents.— Colour  and  Paints.  Pigments,  Oils,  Driers,  Varnishes,  etc.  Tools 
used  by  Painters.  How  to  Mix  Oil  Paints.  Distemper  or  Tempera  Painting, 
Whitewashing  and  Decorating  a Ceiling.  Painting  a Room.  Papering  a Room. 
Embellishment  of  Walls  and  Ceilings. 

Boot  Making;  and  Mending.  Including  Repairing,  Lasting,  and 
Finishing.  With  179  Engravings  and  Diagrams. 

Contents. — Repairing  Heels  and  Half-Soling.  Patching  Boots  and  Shoes. 
Re-Welting  and  Re-Soling.  Boot  IVjlaking.  Lasting  the  Up^er.  Sewing  and 
Stitching.  Making  the  Heel.  Knifing  and  Finishing.  Making  Riveted  Buots 
and  Shoes. 

How  to  Write  Signs,  Tickets,  and  Posters.  With  170  Engravings 
and  Diagrams. 

Contents. — The  Formation  of  Letters,  Stops,  and  Numerals.  The  Sign- 
writer’s  Outfit.  Making  Signboards  and  Laying  Ground  Colours.  The  Simpler 
Forms  of  Lettering.  Shaded  and  Fancy  Lettering.  Painting  a Signboard. 
Ticket-Writing.  Poster- Painting.  Lettering  with  Gold,  etc. 

Wood  Finishing.  Comprising  Staining,  Varnishing,  and  Polishing. 
With  Engravings  and  Diagrams. 

Contents.— Vrocossos  of  Finishing  Wood.  Processes  of  Staining  Wood. 
French  Polishing.  Fillers  for  Wood  and  Filling  In.  Bodying  In  and  Spiriting 
Off.  Glazing  and  Wax  Finishing.  Oil  Polishing  and  Dry  Shining.  Re-poli.shing 
and  Reviving.  Hard  Stopping  or  Beaumontage.  Treatment  of  Floors  Stains. 
Processes  of  Varnishing  Wood  Varnishes.  Re-polishing  Shop  Fronts. 
Dyna.mos  and  Electric  Motors.  With  142  Engravings  and  Diagrams 
Contents. — Introduction.  Siemens  Dynamo.  Gramme  Dynamo.  Manchester 
Dynamo.  Simplex  Dynamo,  Calculating  the  Size  and  Amount  of  Wire  for 
Small  Dynamos.  Ailments  of  Small  Dynamo  Electric  Machines  : their  Causes 
and  Cures.  Small  Electro-motors  without  Castings.  ■ How  to  Determine  the 
Direction  of  Rotation  of  a Motor.  How  to  Make  a Shuttle-Armature  Motor. 
Undertype  50-Watt  Dynamo.  Manchester  Type  440-Watt  D3mamo. 

Cycle  Building  and  Repairing.  With  142  Engravings  and  Diagrams. 

Contents. — Introductory,  and  Tools  Used.  How  to  Build  a Front  Driver, 
Building  a Rear-driving  Safety.  Building  Tandem  Safeties.  Building  Front- 
driver  I'ricycle.  Building  a Hand  Tricycle.  Brazing.  How  to  Make  and  Fit 
Gear  Cases.  Fittings  and  Accessories.  Wheel  Making.  Tyres  and  Methods 
of  Fixing  them.  Enamelling.  Repairing. 

Decorative  Designs  of  All  Ages  for  All  Purposes.  With  277 

Engravings  and  Diagrams. 

Contents. — Savage  Ornament,  Egyptian  Ornament.  Assyrian  Ornament 
Greek  Ornament.  Roman  Ornament.  Early  Christian  Ornament.  Arabic 
Ornament.  Celtic  and  Scandinavian  Ornaments.  Mediaeval  Ornament. 
Renascence  and  Modern  Ornaments.  Chinese  Ornament.  Persian  Ornament. 
Indian  Ornament.  Japanese  Ornament. 

Mounting  a.nd  Framing  Pictures.  With  240  Engravings,  etc. 

Contents. — Making  Picture  Frames,  Notes  on  Art  Frames.  Picture  Frame 
Cramps.  Making  Oxford  Frames.  Gilding  Picture  Frames.  Methods  of 
Mounting  Pictures.  Making  Photograph  Frames.  Frames  covered  with  Plush 
and  Cork.  Hanging  and  Packing  Pictures. 

Smiths’  Work.  With  21 1 Engravings  and  Diagrams. 

Contents. — Forges  and  Appliances.  Hand  Tools.  Drawing  Down  and  Up- 
setting Welding  and  Punching.  Conditions  of  Work  : Principles  of  Forma- 
tion. lieading  and  Ring  Making.  Miscellaneous  Examples  of  Forged  Work. 
Cranks,  Model  Work,  and  Die  Forging.  Home-made  Forges.  The  Manipula- 
tion of  Steel  at  the  Forge.  (Continued  on  next  page.) 

DAVID  McKAY,  Publisher.  604-608  South  Washington  Square,  Philadelphia. 


HANDICRAFT  SERIES  (continued). 


Glass  Working  by  Heat  and  Abrasion.  With  300  Engravings 
and  Diagrams. 

Contents. — Appliances  used  in  Glass  Blowing.  Manipulating  Glass  Tubing. 
Blowing  Bulbs  and  Flasks.  Jointing  Tubes  to  Bulbs  forming  Thistle  Funnels, 
etc.  Blowing  and  Etching  Glass  Fancy  Articles  ; Embossing  and  Gilding  Flat 
Surfaces.  Utilising  Broken  Glass  Apparatus  ; Boring  Holes  in,  and  Riveting 
Glass.  Hand-working  of  Telescope  Specula.  Turning,  Chipping,  and  Grinding 
Glass.  The  Manufacture  of  Glass. 

Building  Model  Boats.  With  168  Engravings  and  Diagrams. 

Contents. — Building  Model  Yachts.  Rigging^  and  Sailing  Model  Yachts. 
Making  and  Fitting  Simple  Model  Boats.  Building  a Model  Atlantic  Liner. 
Vertical  Engine  for  a Model  Launch.  Model  Launch  Engine  with  Reversing 
Gear.  Making  a Show  Case  for  a Model  Boat. 

Electric  Bells,  How  to  Make  and  Fit  Them.  With  162  En- 
gravings and  Diagrams. 

Contents. — The  Electr  c Current  and  the  Laws  that  Govern  it.  Current 
Conductors  used  in  Electric-Bell  Work.  Wiring  for  Electric  Bells.  Elaborated 
Systems  of  Wiring ; Burglar  Alarms.  Batteries  for  Electric  Bells.  The  Con- 
struction of  Electric  Bells,  Pushes,  and  Switches.  Indicators  for  Electric-Bell 
Systems. 

Bamboo  Work.  With  177  Engravings  and  Diagrams. 

Contents. — Bamboo  ; Its  Sources  and  Uses.  How  to  Work  Bamboo.  Bamboo 
Tables.  Bamboo  Chairs  and  Seats.  Bamboo  Bedroom  Furniture.  Bamboo 
Hall  Racks  and  Stands.  Bamboo  Music  Racks.  Bamboo  Cabinets  and  Book- 
cases. Bambco  Window  Blinds.  Miscellaneous  Articles  of  Bamboo.  Bamboo 
Mail  Cart. 

Taxidermy.  With  108  Engravings  and  Diagrams. 

Skinning  Birds.  Stuffing  and  Mounting  Birds.  _ Skinning  and 
Stuffing  Mammals.  Mounting  Animals’  Horned  Heads  : Polishing  and  Mount- 
ing Horns.  Skinning,  Stuffing,  and  Casting  Fish.  Pieserving,  Cleaning,  and 
Dyeing  Skins.  Preserving  Insects,  and  Birds’  Eggs.  Cases  for  Mounting 
Specimens. 

Tailoring:.  With  180  Engravings  and  Diagrams. 

Contents. — Tailors’  Requisites  and  Methods  of  Stitching.  Simple  Repairs 
and  Pressing.  Relining,  Repocketing,  and  Recollaring.  How  to  Cut  and 
Make  Trousers.  How  to  Cut  and  Make  Vests.  Cutting  and  Making  Lounge 
and  Reefer  Jackets.  Cutting  and  Making  Morning  and  Frock  Coats. 
Photog:raphic  Cameras  and  Accessories.  Comprising  How  to 
Make  Cameras,  Dark  Slides,  Shutters,  and  Stands.  With  160 
Illustrations. 

Contents. — Photographic  Lenses  and  How  to  Test  them.  Modern  Half-plate 
Cameras.  Hand  and  Pocket  Cameras.  Ferrotype  Cameras.  Stereoscopic 
Cameras.  Enlarging  Cameras.  Dark  Slides.  Cinematograph  Management. 

Optical  Lanterns.  Comprising  The  Construction  and  Management 
OF  Optical  Lanterns  and  the  Making  of  Slides.  With  160 
Illustrations. 

Contents. — Single  Lanterns.  Dissolving  View  Lanterns.  Illuminant  for 
Optical  Lanterns.  Optical^  Lantern  Acces.sories.  Conducting  a Limelight 
Lantern  Exhibition.  Experiments  with  Optical  Lanterns.  Painting  Lantern 
Slides.  Photographic  Lantern  Slides.  Mechanical  Lantern  Slides.  Cinemato- 
graph Management. 

Eng:raving‘  Metals.  With  Numerous  Illustrations. 

Contents.  — Introduction  and  Terms  used,  £ngraver^’  Tools  and  their  Uses. 
Elementary  Exercises  in  Engraving.  Engraving  Plate  and  Precious  Metals. 
Engraving  Monograms.  Transfer  Processes  of  Engraving  Metals.  Engraving 
Name  Plates.  Engraving  Coffin  Plates.  Engraving  Steel  Plates.  Chasing 
and  Embossing  Metals.  Etching  Metals. 

Basket  Work.  With  189  Illustrations. 

Contents. — Tools  and  Materials.  Simple  Baskets.  Grocer’s  Square  Baskets. 
Rouna  Baskets.  Oval  Baskets.  Flat  Fruit  Baskets.  Wicker  Elbow  Chairs. 
Baske‘  Oottle-casings.  Doctors’  and  Chemists’  Baskets.  Fancy  Basket  Work. 
Sussex  frug  Basket.  Miscellaneous  Basket  Work.  Index 

DAVID  McKAY,  Publisher,  604-608  South  Washington  Square,  Philadelphia. 


HANDICRAFT  SERIES  (continued). 


Bookbinding . With  125  Engravings  and  Diagrams. 

Contents. — Bookbinders*  Appliances.  Folding  Printed  Book  Sheets.  Beat- 
ing and  Sewing.  Rounding,  Backing,  and  Cover  Cutting.  Cutting  Book  Edges. 
Covering  Books.  Cloth-bound  Books,  Pamphlets,  etc.  Account  Books, 
Ledgers,  etc.  Coloring,  Sprinkling,  and  Marbling  Book  Edges.  Marbling 
Book  Papers.  Gilding  Book  Edges.  Sprinkling  and  Tree  Marbling  Book 
Covers.  Lettering,  Gilding,  and  Finishing  Book  Covers.  Index. 

Bent  Iron  Work.  Including  Elementary  Art  Metal  Work.  With 
269  Engravings  and  Diagrams. 

Contents. — Tools  and  Materials.  Bending  and  Working  Strip  Iron.  Simple 
Exercises  in  Bent  Iron.  Floral  Ornaments  for  Bent  Iron  Work.  Candlesticks. 
Hall  Lanterns.  Screens,  Grilles,  etc.  Table  Lamps.  Suspended  Lamps  and 
Flower  Bowls.  Photograph  Frames.  Newspaper  Rack.  Floor  Lamps. 
Miscellaneous  Examples.  Index. 

Photography.  With  70  Engravings  and  Diagrams. 

Contents. — The  Camera  and  its  Accessories.  The  Studio  and  Darkroom. 
Plates.  Exposure.  Developing  and  Fixing  Negatives.  Intensification  and 
Reduction  of  Negatives.  Portraiture  and  Picture  Composition.  Flashlight 
Photography.  Retouching  Negatives.  Processes  of  Printing  from  Negatives. 
Mounting  and  Finishing  Prints.  Copying  and  Enlarging.  Stereoscopic 
Photography.  Ferrotype  Photography.  Index. 

Upholstery.  With  162  Engravings  and  Diagrams. 

Contents. — -Upholsterers’  Materials.  Upholsterers’  Tools  and  Appliances. 
Webbing,  Springing,  Stuffing,  and  Tufting.  Making  Seat  Cushions  and  Squabs. 
Upholstering  an  Easy  Chair.  Upholstering  Couches  and  Sofas.  Upholstering 
Footstools,  Fenderettes,  etc.  Miscellaneous  Upholstery.  Mattress  Making 
and  Repairing.  Fancy  Upholstery.  Renovating  and  Repairing  Upholstered 
Furniture.  Planning  and  Laying  Carpets  and  Linoleum.  Index. 

Leather  Working.  With  152  Engravings  and  Diagrams. 

Contents. — Qualities  and  Varieties  of  Leather.  Strap  Cutting  and  Making. 
Letter  Cases  and  Writing  Pads.  Hair  Brush  and  Collar  Cases.  Hat  Cases. 
Banjo  and  Mandoline  Cases.  Bags.  Portmanteaux  and  Travelling  Trunks. 
Knapsacks  and  Satchels.  Leather  Ornamentation.  Footballs.  Dyeing 
Leather.  Miscellaneous  Examples  of  Leather  Work.  Index. 

Harness  Making.  With  197  Engravings  and  Diagrams. 

Contents. — Harness  Makers’  Tools.  Harness  Makers’  Materials.  Simple 
Exercises  in  Stitching.  Looping.  Cart  Harness.  Cart  Collars.  Cart  Saddles. 
1^'ore  Gear  and  Leader  Harness.  Plough  Harness.  Bits,  Spurs,  Stirrups,  and 
Harness  Furniture.  Van  and  Cab  Harness.  Index. 

Sad  cilery.  With  99  Engravings  and  Diagrams. 

Contents. — Gentleman’s  Riding  Saddle.  Panel  for  Gentleman’s  Saddle 
Ladies’ Side  Saddles.  Children’s  Saddles  or  Pilches.  Saddle  Cruppers,  Breast- 
plates, and  other  Accessories.  Riding  Bridles.  Breaking-down  Tackle  Head 
Collars.  Horse  Clothing.  Knee-caps  and  Miscellaneous  Articles.  Repairing 
Harness  and  Saddlery.  Re-lining  Collars  and  Saddles.  Riding  and  Driving 
Whips.  Superior  Set  of  Gig  Harness.  Index. 

Knotting  and  Splicing,  Ropes  and  Cordage.  With  208 

Engravings  and  Diagrams. 

Contents.— Introduction.  Rope  Formation.  Simple  and  Useful  Knots. 
Eye  Knots,  Hitches  and  Bends.  Ring  Knots  and  Rope  Shortenings,  'lies 
and  Lashings.  Fancy  Knots.  Rope  Splicing.  Working  Coroage.  Ham- 
mock Making.  Lashings  and  Ties  for  Scaffolding.  Splicing  and  Socketing 
Wire  Ropes.  Index. 

Beehives  and  Beekeepers’  Appliances.  With  155  Engravings 

and  Diagrams. 

—Introduction.  A Bar-Frame  Beehive.  Temporary  Beehive. 
Tiering  Bar-Frame  Beehive.  The  “ W.  B.  C.”  Beehive.  Furnishing  and 
Stocking  a Beehive.  Observatory  Beehive  for  Permanent  Use,  Observatory 
Beehive  for  Temporary  Use.  Inspection  Case  for  Beehives.  Hive  for  Rear- 
ing Queen  Bees.  Super-Clearers,  Bee  Smoker.  Honey  Extractors.  Wax 
Extractor.^-  Beekeepers'  Miscellaneous  Appliances.  Index. 

DAVID  McKAY,  Publisher,  604-608  South  Washington  Square.  Philadelphia. 


PHOTOGRAPHIC 

CHEMISTRY 


With  Numerous  Engravings  and  Diagrams 


EDITED  BY 

PAUL  N.  HASLUGK 


PHILADELPHIA 

DAVID  McKAY,  Publisher 
610  South  Washington  Square 
1916 


\ 


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HHE  GETTY  RESEARCH 


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PUBLISHERS’  NOTE 


This  short  treatise  on  Photographic  Chemistry  is  issued 
in  the  confident  belief  that  it  is  not  only  thoroughly 
practical  and  reliable,  but  is  so  simply  worded  that 
even  inexperienced  readers  can  understand  it.  Should 
anyone,  however,  encounter  unexpected  difficulty,  he 
has  only  to  address  a question  to  the  Editor  of  Work, 
La  Belle  Sauvage,  London,  E.C.,  and  his  query  will 
be  answered  in  the  columns  of  that  journal. 


RESEARCH  LIBRARY 
THE  GETTY  RESEARCH  INSTITUTE 


OHN  MOORE  ANDREAS  COLOR  CHEMISTRY  LIBRARY  EOUNDATION 


CONTENTS 


chapter  page 

I. — Introductory  : Relation  of  Chemistry  to 

Photography 9 

II. — Some  Fundamental  Chemical  Laws  . , 24 

III.  — Meaning  of  Symbols  and  Equations  . . 35 

IV.  — Water,  its  Properties  and  Impurities  . . 44 

V. — Oxj^gen  and  Hydrogen  Photographically 

Considered 48 

VI. — Theories  Concerning  the  Latent  Image  . 56 


VII. — Chemistry  of  Development,  Toning,  In- 


tensification, etc. 68 

VIII. — Nitrogen  Compounds  Employed  in  Photo- 
graphy . 89 

IX. — The  Halogens  and  Haloid  Salts  . . .103 

X. — Sulphur  and  its  Compounds  . . .114 

XI. — Metals,  Alkali  Metals,  etc 122 

XII.— Organic  or  Carbon  Compounds  Used  in 

Photography 134 

XIII. — Pyroxyline,  Albumen,  Gelatine,  etc.  . . 143 

XIV.  — Benzene  and  the  Organic  Developers.  . 148 

Index 157 


LIST  OF  ILLUSTRATIONS 


Fig.  page 

1. — Apparatus  for  Experiment  to  Prove  Indestructibility 

of  Matter 12 

2. — Heating  Glass  Tubing  Previous  to  Bending  , , .13 

3. — Cork  Borers 14 

4.  — Sharpener  for  Cork  Borers 14 

5.  — Method  of  Folding  Filter  Paper 15 

6— Arrangement  of  Apparatus  for  Filtration  ....  15 

7. — Wash  Bottle 16 

8. — Method  of  Filtering  Acids 16 

9. — Evaporating  Basin 16 

10. — Bunsen  Burners 17 

11. — Bending  Wire  for  Tripod 17 

12. — Apparatus  for  Evaporation 17 

13.  — Water  Bath  with  Copper  Funnel-holder  . . . .18 

14. — Liebig’s  Condenser 19 

15.  — Apparatus  for  Purifying  Methylated  Spirit  . . .20 

16. — Apparatus  for  Sublimation  Experiment  . . . .22 

17. — The  Process  of  Electrolysis 32 

18. — Glass  Heating  Flask  . 48 

19. — Wide-necked  Glass  Flask  48 

20. — Thistle  Funnel  . . 49 

21.  — Stoppered  Retort 49 

22. — Test  Tube  and  Holder 49 

23.  — Method  of  Preparing  Oxygen  ; 50 

24.  — Preparation  of  Hydrogen 53 

25. — Diagram  Explaining  Molecuiar  Strain  Hypothesis  . . 64 

26. — Acid  Development 74 

27. — Alkaline  Development 74 

28. — Preparation  of  Nitric  Acid  . 92 

29. — Preparation  of  Ammonia 95 

30. — Preparation  of  Chlorine 104 

31. — Experiment  Showing  Action  of  Light  on  Silver  Chloride  106 


PHOTOGRAPHIC  CHEMISTRY 


CHAPTER  I. 

INTRODUCTION  : RELATION  OF  CHEMISTRY  TO 
PHOTOGRAPHY. 

Preliminary. — Photography  is  essentially  a branch 
of  practical  chemistry.  The  materials  used  and 
the  changes  brought  about  by  their  agency  are  all 
subject  to  chemical  laws.  Of  course,  chemistry 
does  not  pretend  to  explain  all  the  changes  taking 
place  in  photographic  operations,  because  many 
of  them  are  still  matters  for  scientifie  investiga- 
tion, such  as,  for  example,  the  exact  composition 
of  the  compound  produced  by  exposing  silver 
chloride  to  the  action  of  light.  In  this  handbook 
it  is  intended  to  give  such  information  as  will 
assist  the  photographer  in  gaining  a knowledge  of 
the  chemical  composition  and  properties  of  his 
materials,  and  of  the  probable  changes  brought 
about  by  their  employment  in  his  profession. 

Aim  of  Chemistry. — The  photographer  is  con- 
scious of  innumerable  changes  continually  taking 
place  in  his  materials  and  reagents.  Developing 
solutions  gradually  turn  brown ; around  the  top 
of  the  ammonia  bottle  a white  crystalline  mass 
gradually  forms;  and  if  the  supply  of  sodium 
sulphite  is  exposed  to  the  air  for  a considerable 
time,  it  will  lose  its  transparency  and  be  converted 
into  a white  opaque  mass,  no  longer  suitable  for 
photographic  operations.  These  incidents,  and 
many  others  of  a similar  nature,  are  of  frequent 


10 


PHOTOGRAPHIC  CHEMISTRY. 


occurrence.  The  aim  of  science  is  to  investigate 
these  innumerable  transformations  and  to  en- 
deavour to  explain  them.  As  can  readily  be 
understood,  the  domain  of  science  is  so  vast  that, 
for  the  sake  of  convenience,  it  is  divided  into  cer- 
tain branches.  In  this  manner  have  arisen  the 
sciences  of  chemistry,  physics,  electricity,  etc. 
Most  of  these  sciences  gradually  merge  into  one 
another,  so  that  it  is  difficult  in  some  cases  to 
say  definitely  where  one  science  begins  and  another 
ends.  This  is  especially  so  in  the  case  of  physics 
and  chemistry,  and  it  is  important  to  have  a very 
clear  idea  of  the  distinction  between  these  two 
sciences. 

Distinction  between  Chemistry  and  Physics. — 
If  a piece  of  glass  rod  is  rubbed  with  a dry 
cloth  it  is  found  that  the  glass  has  acquired  a 
remarkable  property  of  attracting  light  articles, 
such  as  pieces  of  paper.  A piece  of  iron  sus- 
pended vertically  for  some  time  gradually  ac- 
quires the  property  of  attracting  small  pieces  of 
iron,  becoming,  in  fact,  a magnet.  In  both  cases 
the  glass  and  iron  have  undergone  no  perceptible 
change;  the  glass  is  still  glass  and  the  iron  is 
iron.  Again,  heat  converts  water  into  steam,  and 
on  subsequent  cooling  this  passes  back  again  to 
water ; if  it  is  submitted  to  a lower  temperature 
it  is  converted  to  ice.  But  during  these  transi- 
tions from  the  gaseous,  liquid,  and  solid  condi- 
tions the  matter  composing  the  steam,  water,  and 
ice  has  undergone  no  change.  All  such  changes  in 
the  condition  of  bodies,  unaccompanied  by  any 
real  alteration  in  substance,  are  spoken  of  as 
physical  phenomena,  and  their  consideration  and 
study  belong  to  the  science  of  physics.  It  is  well 
known  that  a piece  of  bright  iron  soon  tarnishes 
in  a moist  atmosphere,  and  gradually  becomes 
covered  with  brown  scales,  a change  we  term  rust- 
ing. On  examination,  this  iron  rust  is  found  to 
be  entirely  different  from  the  original  bright  iron. 


INTRODtJCTOIlY. 


11 


and  to  convert  it  back  again  to  its  former  state 
would  be  a lengthy  chemical  process.  If  some 
finely  divided  copper  is  mixed  with  powdered  sul- 
phur a greenish-grey  powder  is  produced.  If 
this  powder  is  examined  under  the  microscope  the 
red  particles  of  copper  can  easily  be  distinguished 
from  the  yellow  of  the  sulphur.  If  the  powder 
is  shaken  with  water,  the  sulphur,  owing  to  its 
lightness,  is  easily  washed  away,  leaving  the 
heavier  particles  of  copper  behind.  Or  by  treat- 
ing the  mixture  wdth  carbon  disulphide,  a liquid 
in  which  the  sulphur  is  easily  soluble,  its  separa- 
tion can  be  readily  effected.  If,  however,  this 
mixture  is  heated  in  a test  tube,  it  commences  to 
glow,  and  on  cooling  a black  mass  remains.  This 
black  mass,  on  examination,  is  found  to  differ  in 
all  respects  from  the  original  copper  and  sulphur. 
Under  the  strongest  microscope  obtainable,  not 
a particle  of  copper  or  sulphur  can  be  distin- 
guished. Washing  with  Vvater  or  treatment  with 
carbon  disulphide  will  not  effect  in  any  way  the 
separation  of  the  ingredients.  Evidently  a much 
deeper  change  has  taken  place  here  than  in  the 
case  of  the  physical  changes.  Such  occurrences, 
therefore,  as  in  the  rusting  of  iron,  and  the  heat- 
ing of  copper  and  sulphur  together,  whereby  a 
complete  and  entire  alteration  takes  place,  belong 
to  the  science  of  chemistry.  The  province  of 
chemistry,  then,  is  the  consideration  of  changes 
in  composition,  of  a comparatively  permanent 
character,  which  substances  undergo  under  differ- 
ent conditions. 

Special  Application  to  Fhotography. — It  is 
only  necessary  to  consider  the  foregoing  statement 
for  a short  time  in  order  to  see  how  intimately 
the  science  of  chemistry  is  bound  up  with  that  of 
photography.  Consider  the  numerous  changes 
brought  about  by  the  photographer,  such  as  those 
of  development,  intensifying,  fixing,  toning,  etc. 
He  is  really  bringing  about  a series  of  chemical 


12 


PHOTOGRAPHIC  CHEMISTRY. 


reactions,  and  it  is  evident  that  a proper  under- 
standing of  these  operations  can  only  be  arrived 
at  by  a knowledge  of  chemistry.  A photographer 
who  understands  the  why  and  the  wherefore  of 
the  presence  of  each  compound  in  his  developer, 
or  toning  bath,  is  evidently  better  equipped  than 
the  person  who  is  lacking  in  such  knowledge  and 
works  by  mere  rule  of  thumb.  In  fact,  it  may 


Fig.  1. — Apparatus  for  Experiment  to  Prove  Indestructi- 
bility of  Matter. 

be  said  that  not  only  is  chemistry  necessary  for 
the  photographer,  but  the  future  advancement  of 
photography  will  depend  upon  the  proper  recog- 
nition and  application  of  scientific  principles. 

Indestructihility  of  Matter. — During  many 
chemical  operations  the  material  entering  into  the 
reaction  is,  apparently,  destroyed.  For  instance, 
the  night-light  or  candle,  in  the  ruby  lamp,  after 
being  ignited,  gradually  burns  away.  Has  the 


INTRODUCTORY. 


13 


matter  composing  the  candle  been  destroyed 
during  the  combustion  1 The  answer  is  in  the 
negative.  This  statement  can  be  experimentally 
verified  in  the  following  manner 

An  ordinary  lamp  chimney  is  procured,  and 
into  the  bottom  is  inserted  a perforated  cork  to 
which  a piece  of  candle  is  attached.  At  the  top 
of  the  chimney  is  a loose  plug  of  wire  gauze 
containing  a few  pieces  of  quicklime  and  caustic 
soda.  The  lamp  chimney,  with  its  contents,  is 


Fig.  2. — Heating  Glass  Tubing  Previous  to  Bending. 


then  attached  to  one  arm  of  a balance,  as  in 
Fig.  1,  and  carefully  counterpoised.  The  candle 
is  then  withdrawn  and  ignited  and  is  reintro- 
duced into  the  chimney. 

As  the  candle  burns  and  decreases  in  size  the 
products  of  combustion  are  absorbed  by  the  quick- 
lime, and  the  arm  of  the  balance  to  which  the 
chimney  has  been  suspended  gradually  descends, 
thus  showing  an  increase  in  weight,  and  not  a 
decrease,  as  would  be  the  case  if  the  material  of 
the  candle  were  destroyed  during  combustion. 
From  this  experiment,  and  others  of  a similar 
nature,  it  is  proved  that  matter  is  indestructible. 


14 


PHOTOGRAPHIC  CHEMISTRY. 


The  methods  described  below  will  be  useful  for 
making  the  simple  apparatus  required  in  various  i 
experiments  illustrating  the  elementary  facts  and  c 
principles  of  chemistry.  1 

Bending  Glass  Tubing. — To  do  this  satisfac-  f 
torily,  it  is  essential  that  a sufficient  length  of  J 
tubing  should  be  heated  before  attempting  to  bend 
it.  This  is  readily  attained  by  the  use  of  an 
ordinary  fish-tail  burner.  The  tube  is  introduced 


Fig.  3. — Cork  Borers. 


into  the  top  part  of  the  flame,  at  the  place  where 
the  bend  is  desired,  and  slowly  rotated  (see  Fig. 

2).  As  soon  as  the  requisite  pliability  has  been 
attained,  it  is  removed  from  the  flame  and  bent  | 
to  the  desired  angle.  | 

Boring  a Cork. — The  end  of  the  glass  tubing  I 
is  gently  pressed  upon  the  surface  of  the  cork, 
so  as  to  leave  a slight,  but  decided,  impression. 

A cork  borer  is  then  chosen  which  just  fits  this 
impression ; this  is  slowly  forced  into  the  cork  and 
turned  in  the  opposite  direction  to  that  in  which  ! 
the  cork  is  being  slowly  rotated.  Fig.  3 shows  > 
a set  of  cork  borers  of  different  sizes,  and  Fig.  4 I 


Fig.  4. — Sharpener  for  Cork  Borers.  ! 

I 

a sharpener  for  the  same.  A hole  may  also  be  j 
made  through  a cork  by  piercing  it  with  a red- 
hot  needle  and  then  completing  the  operation  with 
a rat-tail  file. 

Filtration. — The  operation  of  filtration  con- 
sists of  the  removal  of  suspended  matter  from 
a solution  by  means  of  a porous  substance  such  aa 
blotting  paper,  flannel,  cotton,  etc.  This  is  a 
very  important  operation  to  the  photographer,  as  ! 


INTRODUCTORY. 


15 


many  photographic  processes  depend  for  their  suc- 
cess upon  the  complete  removal  of  solid  particles. 
Developing  and  “ hypo  ” solutions  should  in  all 
cases  be  filtered  if  they  contain  suspended  matter. 
A piece  of  blotting  paper  or  other  absorbent 


material  is  cut  into  a circle ; this  is  then  folded 
as  indicated  in  Fig.  5,  which  shows  the  three 
stages.  One  side  is  now  opened  and  introduced 
into  the  funnel,  the  filtering  apparatus  being  ar- 
ranged as  illustrated  by  Fig.  6.  The  filtered 
liquid  is  termed  the  filtrate.  If  the  residue  on 
the  filter  paper  has  to  be  kept,  it  is  of  the  greatest 


Fig-.  6. — Arrangement  of  Apparatus  for  Filtration. 

importance  to  see  that  it  is  thoroughly  washed 
from  any  adhering  filtrate.  This  washing  is  con- 
veniently performed  by  the  aid  of  a wash  bottle 
(Fig.  7).  A wash  bottle  is  constructed  in  the 
following  manner.  A flask,  or  a bottle  with  a 


16 


PHOTOGRAPHIC  CHEMISTRY. 


fairly  wide  neck,  is  taken  and  is  fitted  with  a cork, 
bored  with  two  holes.  Two  pieces  of  glass  tubing 
are  then  bent  as  shown,  and  introduced  through 
the  cork.  A glass  jet  is  then  made  by  heating  a 


Fig.  8. — Method  of 
Filtering  Acids. 


piece  of  glass  tubing  and  drawing  it  out  as  far 
as  it  will  go,  and  connected  with  the  tube  reaching 
to  the  bottom  of  the  bottle  or  flask  by  means  of  a 
piece  of  indiarubber  tubing. 

The  filtration  of  acids  or  other  corrosive 
liquids  can  be  effected  by  replacing  the  filter  paper 


Fig.  9. — Evaporating  Basin. 


by  pieces  of  broken  glass  or  asbestos  (see  Fig.  8). 
A glass  marble  is  first  introduced  into  the  funnel, 
and  then  a few  pieces  of  broken  glass  are  added 
to  a depth  of  about  ^ in 


INTRODUCTORY. 


17 


Evaporation. — The  operation  of  evaporation  is 
best  carried  out  in  porcelain  basins  (see  Fig.  9), 


Fig.  10. — Bunsen  Burners. 


which  can  be  bought  for  a very  small  outlay.  A 
spirit  lamp  may  be  utilised  as  the  source  of  heat, 


Fig.  11. — Bending  Wire 
for  Tripod. 


Fig.  12. — Apparatus  for 
Evaporation. 


care  being  taken  to  see  that  the  flame  is  exactly 
under  the  centre  of  the  basin.  A small  Bunsen 

B 


18 


PHOTOGRAPHIC  CHEMISTRY. 


burner  (Fig.  10),  attached  by  rubber  tubing  to 
a gas  supply,  is,  however,  to  be  preferred,  on 
account  of  the  greater  ease  with  which  the  heat 
may  be  regulated.  A support  for  the  basin  can 
readily  be  made  in  the  following  manner.  Three 
pieces  of  stout  iron  wire  are  taken  and  bent  as 
shown  by  Fig.  11.  The  six  legs  are  then  bound 
together  with  wire  so  as  to  form  a support  for 
the  basin  (see  Fig.  12).  As  some  compounds,  such 
as  gold  chloride,  readily  decompose  when  evapor- 
ated over  the  naked  flame,  it  is  better  to  make 


Fig.  13. — Water  Bath  with  Copper  Funnel-holde’’ 

use  of  a water  bath.  A tin  mug  or  small  sauce- 
pan slightly  smaller  than  the  diameter  of  the 
basin  can  be  utilised  for  this  purpose.  An  im- 
provement on  this  is  the  apparatus  shown  by 
Fig.  13,  which  is  useful  for  many  photographic 
purposes.  As  will  be  seen,  besides  its  use  for 
evaporation,  it  will  keep  solutions  warm  while 
filtering,  as  required  in  emulsion  making,  etc. 

Distillation. — Many  volatile  substances,  such  as 
water,  alcohol,  benzene,  acetone,  etc.,  may  be  puri- 
fied by  distillation.  The  liquid  is  boiled  in  a 
glass,  or  other  vessel,  and  the  vapour  is  con- 
densed by  conducting  it  into  a tube  surrounded 


INTRODUCTORY. 


19 


by  water  and  known  as  a condenser.  Many  forms 
of  apparatus  can  be  used  for  effecting  distillation, 
a few  of  which  will  now  be  described.  A glass 
retort  or  flask  is  a very  convenient  vessel  for  boil- 
ing the  liquid  in,  but  more  common  articles,  such 


2 


to 


as  a tin  can  or  kettle,  would  in  some  cases  supply 
their  place  equally  well.  In  order  to  condense  the 
vapour  given  off  from  the  boiler,  it  is  made  to  pass 
through  a coil  of  pipe  arranged  in  a bucket 
of  cold  water.  A very  compact  apparatus  for 


20 


PHOTOGRAPHIC  CHEMISTRY. 


bringing  about  condensation  is  a Liebig’s  con- 
denser. This  is  essentially  a glass  tube,  sur- 
rounded by  an  outer  one,  which  is  provided  with 
two  small  tubes  for  the  inflow  and  outflow  of 
water  (see  Fig.  14).  If  possible,  it  is  better  to 


Ph 

a: 


0) 

be 


c3 

P. 

<1 

I 


,bo 


have  the  whole  apparatus,  boiler,  and  condenser 
of  glass,  as  these  are  then  more  easily  kept  in 
a state  of  cleanliness. 

Bistillation  of  Water. — Ordinary  tap  water  is 
unsatisfactory  as  a chemical  solvent,  since  it 


INTRODUCTORY. 


21 


invariably  contains  many  impurities,  which  are 
harmful  from  a photographic  point  of  view.  The 
soluble  chlorides  present  react  with  silver  nitrate 
to  form  a white  insoluble  precipitate  of  silver 
chloride.  The  calcium  and  magnesium  salts  are 
precipitated  as  insoluble  oxalates  in  the  presence 
of  any  oxalate.  These  impurities  are  readily  re- 
moved by  submitting  the  w'ater  to  distillation. 
The  first  pint  or  so  distilling  over  should  be  re- 
jected, as  this  contains  practically  all  the  am- 
monia. By  simply  boiling  the  tap  water  this 
ammonia  is  removed  and  most  of  the  magnesium 
and  calcium  is  precipitated  as  insoluble  carbonate. 

Purification  of  Methylated  Sijirit. — Commer- 
cial wood  spirit  is,  as  a rule,  a very  complex 
mixture,  consisting  of  ethyl  and  methyl  alcohols, 
acetic  acid,  resin,  and  water,  etc.  Impure  wood 
spirit  should  not  be  used  for  washing  plates,  as 
the  water  on  the  wet  plate  precipitates  the  resin 
from  the  spirit,  which  is  then  deposited  on  the 
film.  It  may  be  roughly  purified  in  the  following 
manner.  It  is  first  of  all  introduced  into  a large 
flask  or  retort,  which  is  then  connected  to  the  con- 
densing apparatus.  The  flask  or  retort  is  then  im- 
mersed in  a water  bath — on  no  account  should  the 
spirit  be  heated,  over  a naked  flame.  A large 
saucepan  may  be  utilised  for  this  purpose,  and 
the  spirit  distilled  (see  Fig.  15).  To  the  dis- 
tillate, that  is,  the  liquid  distilling  over,  is  added 
a few  lumps  of  quicklime,  broken  into  small  pieces 
the  size  of  a walnut,  and  the  whole  allowed  to 
stand  for  some  time,  being  well  shaken  at  inter- 
vals. The  spirit  is  next  decanted  off  from  the 
quicklime  into  the  distilling  flask  (any  residue 
from  the  first  distillation  being  removed)  and 
again  distilled.  This  purified  spirit  is  then  suit- 
able for  all  photographic  purposes,  such  as  varnish 
making,  the  drying  of  plates,  etc.  Acetone  may 
be  purified  by  a similar  method  to  that  described 
for  methylated  spirit.  Wet  photographic  plates 


teOTOGRAriilC  CHEMiSTHY. 


are  very  quickly  dried,  by  first  allowing  to  drain, 
and  then  immersing  in  methylated  spirit.  These 
methylated-spirit  washings  may  be  dehydrated 
{i.e.  freed  from  water)  by  treating  with  quick- 
lime, decanting,  and  then  distilling. 

Sublimation. — Many  solid  substances,  when 
heated,  are  more  or  less  readily  converted  into 
vapour  without  undergoing  decomposition,  such 
as  iodine,  mercuric  chloride  (corrosive  sublimate), 
ammonium  chloride,  etc.  When  the  vapour  comes 


Fig.  16. — Apparatus  for  Sublimation  Experiment. 

in  contact  with  a cold  surface  it  is  condensed 
and  reappears  in  the  solid  state.  As  an  example 
of  sublimation,  the  reader  can  perform  the  fol- 
lowing experiment.  A small  basin  is  taken  and 
covered  with  a glass  funnel,  the  shank  of  which 
is  very  loosely  packed  with  a piece  of  cotton-wool. 
A small  quantity  of  iodine  is  introduced  into  the 
basin,  which  is  then  carefully  heated  over  a small 
flame  (see  Fig.  16).  The  iodine  is  very  soon  con- 
verted into  a dark  purple  vapour  which  condenses 
on  the  inside  of  the  funnel  in  black  lustrous 
crystals. 


INTRODUCTORY. 


23 


C rystallisation. — Many  substances  exhibit  a 
tendency  to  crystallise — that  is,  to  assume  a 
regular  geometric  form.  Other  substances,  such  as 
gelatine,  albumen,  and  the  like,  are  devoid  of  any 
such  symmetrical  arrangement.  A third  group  of 
substances,  such  as  the  starches,  exhibit  a distinct 
organised  structure,  as  a rule  of  a globular  nature, 
but  in  no  sense  could  it  be  termed  crystalline. 
Generally  speaking,  every  substance  has  a definite 
crystalline  form,  though  there  are  instances  in 
which  two  distinct  substances  have  the  same  cry- 
stalline shape ; such  substances  are  termed  “ iso- 
morphous.”  In  certain  cases  there  is  a substanee 
occurring  in  two  crystalline  forms;  it  is  then 
said  to  be  “ dimori3hous.”  Crystallisation  is  im- 
portant from  a photographic  standpoint,  as  it 
enables  a large  number  of  soluble  compounds,  such 
as  potassium  dichromate,  washing  soda,  etc.,  to 
be  purified  in  a very  simple  manner.  The  sub- 
stance to  be  purified  by  crystallisation  is  dissolved 
in  hot  water  so  as  to  form  a saturated  solution, 
and  filtered  if  necessary.  The  hot  solution  is  then 
cooled  as  quickly  as  possible.  The  object  of  cool- 
ing quickly  is  to  get  a mass  of  very  fine  crystals, 
because  it  has  been  found  by  experience  that  small 
crystals  are  purer  than  those  of  a larger  size, 
obtained  by  allowing  the  saturated  solution  to 
cool  slowly.  The  supernatant  liquor  is  then 
poured  off  from  the  crystalline  mass,  which  is 
next  washed  with  a little  cold  water  and  allowed 
to  drain. 


CHAPTER  II. 


SOME  FUNDAMENTAL  CHEMICAL  LAWS. 

Chemical  Theory. — It  is  advisable  at  this  stage 
to  consider  a few  definitions  and  some  of  the  more 
important  theories  which  attempt  to  account  for 
chemical  phenomena.  This  portion  of  the  subject 
is  not  directly  applicable  to  photography,  but  as 
it  lies  at  the  root  of  modern  chemistry,  and  as  it 
is  the  purpose  of  this  handbook  to  impart  a sim- 
ple chemical  explanation  of  some  of  the  more 
common  photographic  operations,  it  is  of  extreme 
importance  that  these  principles  should  be 
thoroughly  understood. 

Definition  of  an  Element. — If  a piece  of  chalk 
is  taken  and  very  carefully  analysed,  it  is  found 
to  consist  of  three  distinct  substances,  calcium  (a 
metal),  carbon,  and  oxygen.  If  each  one  of  these 
three  substances  is  taken  and  submitted  to  the 
action  of  the  most  powerful  chemical  reagents,  to 
the  most  exhaustive  operation  of  analysis,  nothing 
is  obtained  from  the  calcium  but  calcium,  from 
the  carbon  but  carbon,  and  from  the  oxj^gen  but 
oxygen.  Each  of  these  three  substances,  then,  con- 
sists of  only  one  kind  of  matter,  and  such  sub- 
stances are  termed  “ elements.’’  Of  course,  as 
the  science  of  chemistry  advances,  and  methods  of 
analysis  become  more  powerful,  it  is  quite  likely 
that  these  so-called  elements  will  be  found  to  be 
of  a complex  nature.  The  number  of  elements 
known  up  to  the  present  time  is  about  seventy- 
eight. 

Metals  and  N on-metals. — If  these  elements  are 
carefully  examined  they  are  found  to  differ  in  a 
very  marked  manner.  In  the  first  place  they 
could  be  grouped  as  gases,  liquids,  and  solids  ; — 


SOME  EXJXDAMENTAL  CHEMICAL  LAWS.  25 


Gases. 

Liquids. 

Solids. 

Oxygen 

Mercury 

Iron 

Hydrogen 

Bromine 

Lead 

Chlorine 

Gold 

Nitrogen 

Sulphur 

Iodine 

Phosphorus,  etc. 

On  further  examination  the  elements  in  the  second 
and  third  columns  in  the  above  table  are  found  to 
be  very  different  in  physical  properties.  Some 
are  bright  and  shiny  in  appearance,  and  are  good 
conductors  of  heat  and  electricity.  Other  ele- 
ments are  dull  in  aspect,  and  are  bad  conductors 
of  heat  and  electricity.  Two  groups  of  elements 
then  are  obtained  : — 


BrirjTit  appearance.  Good 
conductor. 

Dxdl  appearance.  Beal 
conductor. 

Gold 

Copper 

Sulphur 

Iodine 

Lead 

Iron 

Bromine 

Phosphorus 

Silver 

Tin 

Silicon 

Boron 

If  these  two  groups  of  elements  are  studied  they 
are  found  to  differ  very  considerably  both  in 
physical  and  chemical  properties.  For  instance, 
the  first  group  have  the  following  general  pro- 
perties : — 

(1)  They  have  a peculiar  brilliancy  of  surface 
which  is  knowm  as  metallic  lustre. 

(2)  As  a rule,  they  are  good  conductors  of  heat 
and  electricity. 

(3)  They  have  the  power  of  displacing  hydro- 
gen from  acids. 


26 


PHOTOGEAPHIC  CHEMISTRY. 


These  general  properties  are  absent  in  the 
second  group  of  elements.  The  elements  in  the 
first  group  are  known  as  the  metals;  those  in  the 
other  group  are  termed  non-metals. 

Chemical  Comiiounds. — When  two  or  more 
elements  unite  together  to  form  a new  substance, 
having  properties  distinct  from  those  of  its  con- 
stituent elements,  this  new  body  is  termed  a 
chemical  compound. 

Water  is  a chemical  compound,  and  contains 
the  elements  hydrogen  and  oxygen.  By  com- 
paring the  properties  of  water  with  those  of 
hydrogen  and  oxygen,  they  are  seen  to  be  quite 
different.  The  force  which  unites  two  or  more 
elements  together  to  form  a chemical  compound 
is  termed  chemical  affinity  or  attraction. 

Law  of  Consta7it  Froportion. — When  chemical 
elements  combine  together  so  as  to  form  a chemical 
compound,  it  is  found  that  the  proportions  in 
which  they  combine  is  always  a constant  quantity. 
By  making  a quantitative  analysis  of  salt  it  is 
found  to  contain  1 part  of  sodium  to  1*54  parts  of 
chlorine.  No  matter  where  or  how  the  salt  has 
been  obtained — from  the  North  Pole,  the  Equator, 
or  any  other  part  of  the  globe,  provided  it  is 
pure — it  always  contains  these  two  elements  in  this 
proportion,  1 of  sodium  to  1*54  chlorine.  Salt 
has  never  been  obtained  containing  either  a less 
or  greater  quantity  of  its  constituent  elements. 
This  has  been  found  to  hold  true  for  every  chemi- 
cal compound,  whether  procured  artificially  or 
naturall3^  This  fact  has  been  demonstrated  so 
repeatedly  by  chemists  that  it  is  termed  a law, 
and  is  known  as  the  “ Law  of  Constant  Propor- 
tion.” This  is  usually  stated  as  follows  : “ Every 
chemical  compound  contains  the  elements  of  which 
it  is  composed  in  one,  and  only  one,  proportion  by 
weight.” 

Jjaw  of  Multiple  Proportions. — Certain  ele- 
ments combine  together  to  form  more  than  one 


Some  i^undamental  ohemical  laws.  27 


compound.  Copper  combines  with  chlorine  to 
form  two  compounds — cuprous  chloride  and  cupric 
chloride.  Carbon  combines  with  oxygen  to  form 
two  oxides — carbon  monoxide  and  carbon  dioxide. 
The  element  nitrogen  unites  with  oxygen  to  form 
five  distinct  oxides — nitrous  oxide,  nitric  oxide, 
nitrogen  trioxide,  nitrogen  peroxide,  and  nitro- 
gen pentoxide. 

By  making  a careful  analysis  and  study  of  each 
of  the  above  compounds,  they  are  found  to  con- 
tain the  following  weights  of  each  element  : — 


63  grs.  of 

copper  combine  with  35 -5 

grs. 

of  chlorine. 

63  „ 

99 

710 

99 

99 

12  „ 

carbon 

99 

16 

99 

oxygen. 

12  ,, 

59 

99 

32 

99 

99 

14  „ 

nitrogen 

9 9 

8 

99 

99 

14  „ 

99 

16 

99 

14  „ 

99 

99 

24 

99 

99 

14  „ 

99 

99 

32 

99 

99 

14  „ 

99 

99 

40 

99 

On  examining  these  numbers  it  is  seen  that 
the  relative  proportion  of  : 

Chlorine  combining  with  the  same  weight  of  cop- 
per is  1 : 2; 

Oxygen  combining  with  the  same  weight  of  carbon 
is  1 ; 2; 

Oxygen  combining  with  the  same  w^eight  of  nitro- 
gen is  1 : 2 : 3 : 4 : 5. 

Hence,  when  one  element  combines  with  another, 
in  more  than  one  proportion  by  weight,  these 
proportions  bear  a ratio  to  one  another;  the 
higher  proportions  are  always  some  simple  mul- 
tiple of  the  lower  proportions.  This  statement 
is  known  as  the  Law  of  Multiple  Proportions,” 
and  holds  true  for  all  elements  which  form  more 
than  one  compound  with  another  element.  It  is 
important  to  notice  that  these  laws  of  constant 
proportion  and  multiple  proportions  are  not 
theories ; they  are  the  outcome  of  innumerable 
experiments. 


28 


PHOTOGRAPHIC  CHEMISTRY. 


Atomic  Theory. — The  most  satisfactory  ex- 
planation, at  the  present  time,  of  the  fact  that 
substances  always  combine  in  fixed  and  definite 
proportions  by  weight,  or  in  some  simple  multiple 
of  these  proportions,  is  to  be  found  in  the  atomic 
theory.  This  generally  accepted  theory  makes  the 
assumption  that  all  elementary  matter  is  com- 
posed of  exceedingly  minute  particles,  so  small 
that  they  cannot  be  further  subdivided.  These 
indivisible  particles  are  termed  “ atoms.’’  Now, 
according  to  the  atomic  theory,  when  chemical 
combination  takes  place,  it  is  due  to  the  action 
of  atom  on  atom,  or  groups  of  atoms  on  groups 
of  atoms.  That  is,  chemical  action  means  the 
union  or  dissociation  of  these  indivisible  particles. 

Atomic  Weight. — It  follows  that  if  matter  is 
composed  of  atoms,  these  possess  a definite  weight. 
Also,  the  atoms  of  any  one  element  must  be  sup- 
posed to  have  the  same  weight,  while  the  atoms  of 
different  elements  have  different  weights.  Owing 
to  the  extremely  minute  character  of  the  atom,  so 
small  as  to  be  incapable  of  further  subdivision,  it 
is  evident  that  their  absolute  weight  cannot  be 
determined.  All  that  can  be  'done  is  to  obtain 
their  relative  weight  in  terms  "of  some  standard. 
As  the  element  hydrogen  is  the  lightest  known  sub- 
stance, the  atom  of  hydrogen  is  adopted  as  the 
standard  of  comparison.  The  number  so  obtained 
is  termed  the  atomic  weight  of  the  element,  and 
represents  how  many  times  heavier  an  atom  of 
the  element  is  than  the  atom  of  hydrogen.  The 
methods  employed  for  the  determination  of  the 
atomic  weights  will  not  be  discussed  here.  The 
photographer  can  obtain  these  from  any  standard 
work  on  chemistry. 

Explanation^  of  the  Lairs  of  Constamt  Propor- 
tion and  Multiple  Proportion. — If  it  be  granted 
that  elementary  matter  is  made  up  of  atoms,  hav- 
ing definite  weights  and  incapable  of  further 
subdivision,  it  must  follow  that  when  two  ele- 


SOME  FUNDAMENTAL  CHEMICAL  LAWS.  29 


merits  combine,  one  atom  to  one  atom,  the  com- 
bining proportions  (representing  the  weights  of 
the  atoms)  will  always  be  a constant  quantity.  For 
example,  an  atom  of  an  element  A weighing  100 
combines  with  one  atom  of  an  element  B weigh- 
ing 200  to  form  a compound  (A  and  B).  Now  as 
atoms  are  indivisible  (atomic  theory),  it  follows 
that  no  less  quantity  than  200  of  B can  combine 
with  an  atom  of  A.  The  combining  proportions 
will  always  be  100  A : 200  B,  or  1 : 2. 

It  also  follows  that  if  an  element  combines  in 
more  than  one  proportion  by  weight  with  another 
element,  that  the  higher  proportions  will  always 
be  some  simple  multiple  of  the  atomic  weight  of 
the  element  or  lowest  proportion  (Law  of  Multiple 
Proportions).  For  example,  an  element  A having 
the  atomic  weight  of  100,  combines  with  B,  another 
element  whose  atom  weighs  200,  to  form  a series 
of  compounds. 

Now,  according  to  the  theory,  the  compounds 
would  be  : 

A + B A-I-2B  A + 3B,etc. 

1 atom  1 atom  1 atom  -f  2 atoms  1 atom  + 3 atoms 

100  : 200  100  : 400  100  : 600,  etc. 

Or  the  relative  proportion  of  B combining  with 
A would  be  1:2:3,  which  is  in  agreement  with 
the  law  of  multiple  proportions.  The  atomic 
theory  may  or  may  not  be  literally  true,  but  it 
is  the  best  guess  that  has  been  made  to  account  for 
the  fundamental  laws  of  chemical  action,  and  it 
is  a very  convenient  means  for  interpreting  the 
facts  of  chemistry.  It  is  a splendid  working 
hypothesis,  and  has  contributed  very  largely  to 
the  advancement  of  the  science. 

The  Molecule. — By  making  very  careful  in- 
vestigations of  certain  phenomena  of  light,  elec- 
tricity, of  liquid  films,  and  the  conduct  of  gases 
under  varying  conditions,  physicists  have  come  to 
the  conclusion  that  these  phenomena  can  only  be 
explained  on  the  assumption  that  matter  is  made 


30 


PHOTOGEAPHTC  CHEMISTEY. 


up  of  very  minute  particles,  which  are  termed 
molecules.  A molecule  may  be  defined  as  the 
smallest  particle  of  matter  which  can  exist,  as 
such,  in  a free  state.  For  instance,  if  a small 
cube  of  silver  nitrate  were  taken,  it  could  be  cut  in 
half,  this  half  again  halved,  and  so  on  till  a very 
minute  quantity  of  silver  nitrate  was  obtained. 
Let  it  be  imagined  that  this  small  quantity  of 
substance  could  be  further  subdivided  and  sub- 
divided, till  at  last  such  a minute  particle  would 
be  obtained,  that,  if  further  subdivided,  it  would 
break  down  into  its  component  atoms,  silver, 
nitrogen,  and  oxygen.  :This  small  particle  of 
matter,  representing  the  limit  of  subdivision,  is 
termed  a molecule.  Of  course,  this  is  a purely 
theoretical  consideration,  as  no  person  has  ever 
seen  a molecule.  This  molecule  theory,  like  the 
atomic  theory,  is  a good  working  hypothesis,  and 
is  in  harmony  with  most  of  the  observed  facts. 
The  molecules  of  the  elements  generally  consist 
of  two  atoms ; the  molecules  of  compounds  may 
consist  of  any  number  of  atoms,  from  two  up- 
wards. The  molecules  of  water  consist  of  three 
atoms,  two  of  hydrogen  and  one  of  oxygen.  That 
of  silver  nitrate  contains  five  atoms,  one  of  silver, 
one  of  nitrogen,  and  three  of  oxygen.  Many  facts 
have  to  be  taken  into  consideration  before  we 
arrive  at  the  number  of  atoms  a compound  sub- 
stance contains,  and  for  further  elucidation  of 
this  point,  the  reader  is  referred  to  works  on 
chemistry. 

MolecAilar  Weight. — If  the  atomic  weights  of 
the  elements,  multipled  by  the  number  of  atoms 
of  each  element  present  in  a compound,  are  added 
together,  the  number  so  obtained  is  termed  the 
molecular  weight. 

Example  : To  find  the  molecular  weight  of 
silver  nitrate.  This  substance  contains  one  atom 
of  silver,  one  atom  of  nitrogen,  and  three  atoms  of 
oxygen  ; 


SOME  FUNDAMENTAL  CHEMICAL  LAWS.  31 


One  atom  of  silver  weighs  107*7  = 107*7 

One  atom  of  nitrogen  weighs  14*0  = 14*0 

Three  atoms  of  oxygen  weigh  16  x 3 = 48'0 


Molecular  weight  of  silver  nitrate  1697 

Ionic  Theory —ThQVQ  are  many  facts  in  chemis- 
try which  do  not  receive  their  complete  interpreta- 
tion by  means  of  the  atomic  theory  alone,  and 
this  is  especially  so  in  the  case  of  many  dilute 
solutions  of  metallic  salts.  The  ionic  or  elec- 
trolytic dissociation  theory  of  Svante  Arrhenius 
(1887)  offers  an  explanation  of  a number  of  ob- 
scure reactions.  As  the  photographer  is,  for  the 
most  part,  working  with  solutions  during  develop- 
ment, toning,  fixing,  intensifying,  reduction,  etc., 
in  order  to  throw  some  light  on  these  processes,  it 
may  not  be  out  of  place  to  give  a brief  outline  of 
this  ionic  theory. 

Electrolysis. — If  a rod  of  zinc  is  introduced 
into  a solution  containing  dilute  sulphuric  acid, 
hydrogen  is  evolved  and  zinc  sulphate  goes  into 
solution.  If  now  a piece  of  platinum,  a metal 
which  is  not  attacked  by  the  acid,  is  introduced 
into  the  same  liquid,  and  is  connected  with  the 
zinc  by  means  of  a piece  of  wire,  hydrogen  now 
makes  its  appearance  on  the  platinum,  bubbles 
of  which  rise  to  the  surface,  the  zinc  at  the  same 
time  slowly  dissolving  without  any  evolution  of 
gas.  Such  a combination  as  described  is  termed 
a voltaic  cell.  A collection  of  voltaic  cells,  con- 
nected together,  the  platinum  to  the  zinc,  zinc 
to  platinum,  and  so  on,  is  termed  a battery.  If 
the  last  platinum  and  zinc  plates  of  the  battery 
are  connected  together  with  pieces  of  wire,  this 
wire  exhibits  electrical  properties.  From  this 
it  is  evident  that  when  zinc  dissolves  in  dilute 
acid  in  the  presence  of  the  platinum,  electricity 
is  generated.  If  the  two  end  wires  of  the  battery 
are  introduced  into  a solution  of  copper  sulphate 
the  compound  undergoes  decomposition.  The 


32 


PHOTOGRAPHIC  CHEMISTRY. 


copper  is  deposited  on  the  wire  from  the  zinc  end 
of  the  battery,  and  sulphuric  acid  and  oxygen 
make  their  appearance  at  the  wire  from  the 
platinum  end  (see  Fig.  17).  Such  decompositions 
brought  about  by  the  aid  of  electricity  are  termed 
electrolytic  decompositions,  and  the  process  is 
spoken  of  as  one  of  electrolysis. 

Definition  of  Terms. — The  wire  from  the  zinc 
is  charged  with  negative  electricity  and  is  termed 
the  negative  electrode  or  cathode,  and  the  wire 
from  the  platinum  is  charged  with  positive  elec- 


tricity, and  is  termed  the  positive  electrode  or 
anode.  The  anode  is  usually  represented  by  the 
sign  + and  the  cathode  by  — . The  substance 
undergoing  electrolysis  is  termed  an  electrolyte. 
The  electrolysis  of  copper  sulphate  results  in  the 
separation  of  molecules  of  copper  and  molecules 
of  SO4,  which  are  deposited  at  the  cathode  and 
anode  respectively.  These  wandering  molecules 
are  termed  ions.  Those  making  their  appearance 
at  the  anode  are  termed  anions,  and  those  at  the 
cathode  cations.  All  the  metals,  with  hydrogen. 


SOME  FUNDAMENTAL  CHEMICAL  LAWS.  33 


are  cations,  and  the  non-metals  anions.  The  elec- 
trolysis of  copper  sulphate  might  be  represented 
as  follows  : — 

Cation  Anion 

+ ' 

=Cu  i>  SO4 


2SO,  X 2H2O  = H,SO,  + O, 

The  ion  SO4  is  very  unstable,  and  is  decomposed 
by  water  as  indicated  above.  According  to  the 
ionic  theory  electrolytes  do  not  exist  as  such  in 
aqueous  solution — that  is,  they  undergo  ionisation, 
and  break  down  at  once  into  ions,  the  anions  being 
charged  with  negative  and  the  cations  with  posi- 
tive electricity.  Consequently,  in  a solution  of 
silver  nitrate,  instead  of  having  molecules  of 
AgNOg,  cations  of  silver  and  anions  of  NO3  are 
produced.  The  neutral  salts,  such  as  NaCl,  KI, 
NH^Br,  AgNOg,  etc.,  are  those  which  undergo 
ionisation  most  strongly.  The  action  of  silver 
nitrate  on  a solution  of  potassium  bromide  would 
be  represented  on  the  ionisation  theory  in  the  fol- 
lowing way  : — 

+ — -f-  — + — 

Ag  + NO3  + K + Br  = AgBr  + K + NO3 

Ions  of  Ions  of  Silver  bromide  Ions  of 

silver  and  + potassium  = non-ionised  + potassium 
NO3  and  because  it  and  NO3 

bromine.  is  insoluble 

in  water. 

Beactions  of  the  Ions, — The  reactions  used  for 
che  detection  of  substances,  according  to  the  ionic 
theory,  depend  chiefly  on  the  reactions  of  the  ions. 
For  instance,  all  those  compounds  which  in  aque- 
ous solution  produce  the  cation  Fe  (ic)  react  with 
ammonia  to  produce  a brown  flocculent  precipitate 
of  ferric  hydrate.  If  iron  is  present  as  a complex 
ion  {i.e.  with  other  elements)  this  characteristic 
0 


34 


PHOTOGRAPHIC  CHEMISTRY. 


reaction  with  ammonia  is  not  obtained.  For 
instance,  potassium  ferrocyanide  K4Fe(CN)6, 
though  it  contains  an  atom  of  iron  in  the  molecule, 
does  not  produce  a brown  precipitate  on  the  ad- 
dition of  ammonia.  This  is  due  to  the  fact  that 
a solution  of  K^FeCCN)^  undergoes  ionisation, 
forming  potassium  ions  and  complex  ions  of 
Fc(CISr)r,.  It  does  not  produce  free  ions  of  Fe. 

+ ”}■  + "t*  — 

K,Fe(CN)e + K -}-  K + K - - - Fe(CN)e 

Similarly  with  potassium  dichromate.  This  com- 
pound contains  chromium,  having  the  formula 
KoCroO^,  but  does  not  show  the  ordinary  reactions 
for  that  metal.  This  is  du3  to  the  fact  that  its 
solution  in  water  results  'in  the  formation  of 
potassium  ions  and  complex  ions  of  CrgOy. 

K.Cr^O,  — — t>K  + K Cr^O, 

The  few  points  mentioned  above  will  give  the 
photographer  some  idea  of  this  ionic  theory,  which 
of  late  years  has  come  very  much  to  the  front. 
For  a more  detailed  account  reference  must  be 
made  to  some  standard  work  on  physical 
chemistry 


35 


CHAPTER  III. 


MEANING  OF  SYMBOLS  AND  EQUATIONS. 


Chemical  Symbols. — Instead  of  writing  down  each 
time  the  full  name  of  every  element,  certain 
characters  are  used  to  represent  it,  and  these 
characters  are  known  as  symbols.  In  most  cases 
this  is  either  the  first  or  first  two  letters  of  the 
English  or  Latin  name  of  the  element.  Thus  the 
symbol  for  nitrogen  is  N,  for  oxygen  O.  For 
potassium  it  is  K,  being  the  initial  letter  of  the 
Latin  for  potassium,  Kalium.  As  several  of  the 
names  of  the  elements  commence  with  the  same 
letter,  to  prevent  confusion,  other  letters  are 
added  to  the  first  letter,  such  as,  for  instance  : — 

Chlorine  has  symbol  Cl. 

Chromium  ,,  ,,  Cr. 

Cobalt  ,,  ,,  Co. 

Some  of  the  elements  have  as  symbols  two  of  the 
most  significant  letters  of  the  Latin  name  : — 


Mercury  ...  Lat.,  Hydrargyrum 
Gold  ...  „ Aurum 

Silver  ...  ,,  Argentum 

Copper  ...  „ Cuprum 

A symbol,  however,  stands  for 
than  the  mere  abbreviation  of 


Symbol,  Hg 
„ Au. 

» Ag. 

„ Cu. 

something  more 
the  name  of  an 


element.  It  represents  not  only  some  particular 
element,  but  denotes  at  the  same  time  one  atom 
of  the  substance  and  a quantity  of  it  equal  in 
amount  to  the  atomic  weight.  Thus  : — 


The  symbol  Ag  means  1 atom  of  silver  and  107*7 
parts  by  weight; 

The  symbol  Cu  means  1 atom  of  copper  and  63*5 
parts  by  weight; 

The  symbol  O means  1 atom  of  oxygen  and  16 
parts  by  weight. 


36 


PHOTOGEAPHIC  CHEMISTRY. 


Molecular  'Formulae, — To  denote  the  composi- 
tion of  a compound,  the  symbols  of  the  elements 
composing  it  are  written  side  by  side.  If  more 
than  one  atom  of  each  element  is  present  they  are 
denoted  by  placing  a small  number,  termed  the 
exponent,  at  the  bottom  of  the  symbol  on  the 
right-hand  side.  The  expression  is  termed  a 
formula,  and  it  also  denotes  that  the  elements  pre- 
sent in  the  compound  are  chemically  combined. 

The  formula  for  silver  nitrate  is  AgNOg ; 

,,  ,,  ,,  gold  chloride  is  AuClg ; 

,,  ,,  ,,  cane  sugar  is  C12H22O11; 

,,  ,,  ,,  silver  chloride  is  AgCl. 

As  a general  rule,  the  molecules  of  the  elements 
contain  two  atoms,  and  they  are  represented  by 
the  following  molecular  formulae  : 

N2  O2  H2  CI2  Bra  CUa. 

To  denote  two  or  more  molecules  of  a certain  sub- 
stance, a large  number,  termed  the  coefficient,  is 
placed  in  front  of  the  molecular  formula.  Thus 
four  molecules  of  silver  chloride  would  be  repre- 
sented by  4AgCl.  Elements  placed  in  brackets 
are  multiplied  by  the  exponent  at  the  right-hand 
bottom  corner,  in  order  to  obtain  the  total  num- 
ber of  atoms  present  in  the  molecule  : 
(NHJaSO,  or  NaH^SO,. 

Valency. — If  the  formulae  of  the  chlorides  of 
the  elements  are  examined,  they  are  found  to 
differ,  in  a large  number  of  cases,  in  the  number  of 
atoms  of  chlorine  contained  in  the  molecules. 
For  instance,  the  chlorides  of  silver,  copper,  gold, 
and  platinum  have  the  following  molecular  for- 
mulae : Silver  chloride  AgCl. 

Copper  chloride  CuCE. 

Gold  chloride  AuClg. 

Platinic  chloride  PtCE. 

Hence  it  is  seen  that  atoms  of  different  elements 
differ  in  their  pov/er  of  holding  other  elements  in 


MEANING  OF  SYMBOLS  AND  EQUATIONS.  37 

combination.  The  combining  power  of  an  element 
is  measured  in  terms  of  the  combining  power  oi 
hydrogen,  which  is  taken  as  unity.  The  number 
so  obtained  is  termed  the  “ valency  ’’  of  that  par- 
ticular substance.  For  examiDle,  one  atom  of 
hydrogen  combines  v/ith  one  atom  of  chlorine,  one 
atom  of  bromine,  and  one  atom  of  iodine  to  form 
respectively  hydrochloric,  hydrobromic,  and 
hydriodic  acids. 

HCl  HBr  HI 

It  is  evident  from  this  that  chlorine,  bromine, 
and  iodine  have  the  same  combining  power  as 
hydrogen ; and  they  have  therefore  a valency  of 
one.  Such  elements  are  said  to  be  monovalent  or 
univalent.  If  one  atom  of  any  other  element 
combines  with  one  atom  of  chlorine,  bromine, 
or  iodine,  it  follows  that  they  will  be  also  mono- 
valent, such  as  the  elements  potassium,  sodium, 
lithium,  etc.  If  one  atom  of  an  element  combines 
with  two  atoms  of  chlorine,  such  as  calcium, 
barium,  strontium,  cadmium,  etc.,  they  will  evi- 
dently have  double  the  combining  power  of  hydro 
gen.  These  elements  are  said  to  be  divalent.  Ele- 
ments combining  with  three,  four,  five,  six,  or 
more  atoms  of  chlorine  are  termed  tri-,  tetra-, 
quinque-  (or  penta-),  and  hexa-valent  elements 
respectively.  It  is  impossible  to  write  the  correct 
formula  of  a compound  or  to  express  a chemical 
action  by  means  of  an  equation  unless  the  valency 
of  the  elements  be  known. 

Bonds  to  Represent  Valency. — In  some  cases 
it  is  found  convenient  to  represent  the  valency  of 
an  element  by  drawing  from  the  symbol  a number 
of  lines  or  bonds.  Thus  a monovalent  element  is 
said  to  have  one  bond,  a divalent  element  two 
bonds,  and  so  on.  By  this  means,  representative 
diagrams  may  be  drawm  which  give  a much 
clearer  idea  of  chemical  reactions  than  would 
otherwise  be  possible.  This  is  more  noticeably  the 
case  wdth  complex  organic  comoounds. 


38 


PHOTOGKAPHIC  CHEMISTR-S. 


I 

Cl-  --0-  Au— 

1 

Cl-H  H-O-H  Cl 

I 

Au— Cl 
I 

Cl 

Of  course,  this  is  purely  a symbolical  representa- 
tion of  the  idea  that  each  atom  possesses  a definite 
combining  power,  and  it  must  not  be  supposed 
that  elements  have  little  arms  projecting. 

Compound  Radicals. — So  far,  mention  has  only 
been  made  of  the  valency  of  single  elements.  In 
some  cases  it  is  found  that  groups  of  atoms  take 
the  place  of  mono-,  di-,  etc.,  valent  elements. 
These  groups  are  termed  “ compound  radicals,’’ 
and  are,  as  a rule,  enclosed  in  brackets  when  writ- 
ing the  molecular  formula  of  a compound  con- 
taining them. 

It  may  be  pointed  out  that  a very  common  com- 
pound radical  is  a group  containing  one  atom  of 
nitrogen  and  four  atoms  of  hydrogen.  This  is  a 
monovalent  group,  and  behaves  in  a very  similar 
manner  to  sodium  or  potassium.  In  order  to 
show  this  analogy  it  is  termed  ammonium  (NH4). 

Hydrochloric  acid,  HCl  NaCl  KCl  NH4CI ; 
(Sulphuric  acid,  H2SO4  NagSO.  K2SO4  (NH4)2S04  ; 
Oxalic  acid,  H2C2O4  Na2C204  K2C2O4  (NH4)2C204 

Sodium  Potassium  Ammonium 
salts.  salts  salts. 

A few  other  common  compound  radicals  are  : 
(OH),  hydroxyl,  and  (ON)  cyanogen. 

Variable  Valency. — Certain  elements  form  two 
or  more  classes  of  compounds,  depending  upon 
their  degree  of  oxidation  or  reduction. 

For  example,  mercury  and  copper  each  form 
two  oxides  with  oxygen — mercurous  and  mercuric 
oxide,  cuprous  and  cupric  oxide.  These  have  the 
following  formulie  : — 


MEANING  OF  SYMBOLS  AND  EQUATIONS.  39 


Merciirzc  oxide,  HgO.  Cupric  oxide,  CuO. 

Mercurow.5  oxide,  Hg20.  Cuprows  oxide,  CiuO. 

It  is  evident  from  what  has  already  been  said 
about  valency  that  these  metals,  mercury  and 
copper,  in  the  “ ic  ” condition — that  is  to  say,  in 
the  reduced  state — are  monovalent.  In  the  “ ous  ” 
condition,  or  oxidised  state,  they  are  divalent. 
Owing  to  this  fact,  each  metal  forms  twm  classes 
of  salts,  the  “ ous  ” and  ic  ” compounds.  In  the 
table  on  p.  40  a few  of  the  more  common  metals, 
having  this  variable  valency,  are  given. 

This  variable  valency  is  also  met  with  in  the 
non-metallic  elements.  It  is  readily  seen  if  the 
formulae  of  sulphuretted  hydrogen,  sulphur  di- 
oxide, and  sulphuric  acid  are  w^ritten  out  fully  : — 

O 

II 

H-S-H  0 = S = 0 H-O-S-0— H 

II 

O 

Sulphuretted  hydrogen  Sulphur  dioxide  Sulphuric  acid 
S divalent.  S tetravalent.  S hexavalent. 

Chemical  Equations. — In  dealing  with  cases  of 
chemical  action  it  is  desirable  to  express  the 
change  taking  place  by  means  of  symbols.  A sym- 
bolical representation  of  a chemical  change  is 
termed  an  equation.  The  symbols  or  formulae  of 
the  reacting  bodies  are  placed  on  the  left  hand 
of  the  sign  =,  and  the  resulting  substances  are 
represented  by  formulae  and  symbols,  placed  to  the 
right  of  it.  Taking  the  following  equation  as  an 
example  : — 

H2SO,  + Zn  = ZnSO,  -h  H^, 

this  means  that  one  molecule  of  sulphuric  acid 
acting  upon  or  reacting  with  (represented  by  the 
sign  +)  zinc,  yields,  or  produces  (represented  by 
the  sign  =),  one  molecule  of  zinc  sulphate,  to- 
gether with  (sign  -f)  a molecule  of  hydrogen. 
Also,  it  must  be  noticed  that  both  sides  of  an 
squation  must  balance — that  is,  if  four  atoms  of 


40 


Metal  in  ic  or  Oxidised  Condition 

a* 

C 

c 

c3 

<M  CO  CO 

Chloride. 

SBoSld' 

ojj  = o pt  s 

Wofs^^2<iP^ 

1 Oxide. 

i 

. 

ic  ® a ^ 
MoProq<i1 

O 

P 

OJ 

I-I  I-H  (M  C<>  — 1 (N 

p 

C3 

k 

Q 

d 

r2 

o o o g" 

s 

W Q pH  CC  < Ph 

O 

o 

o 

Oxide. 

1 

0.0  o 1 

bo  c = 1 

WqPho2<1 

cl  O ci  ^ 

ffi  O r-l 


I 


MEANING  OE  SYMBOLS  AND  EQUATIONS.  41 


a particular  element  are  on  the  left-hand  side, 
these  must  be  accounted  for  on  the  right-hand  side. 

The  question  of  valency  must  also  be  considered. 
For  instance,  take  the  following  equation  : — 

AgN03  + 3HC1  = AgCl^  + HNO3. 

This  is  wrong  for  these  reasons  : — 

(a)  Silver  should  be  monovalent,  not  divalent. 
{b)  Three  atoms  of  hydrogen  and  three  atoms 
of  chlorine  are  on  one  side  and  only  two  atoms 
of  chlorine  and  one  atom  of  hydrogen  are  on  the 
other  side.  As  the  silver  is  a monovalent  metal, 
therefore  the  formula  of  the  chlorine  wmald  be 
AgCl.  This  one  atom  of  chlorine  requires  only 
one  molecule  of  hydrochloric  acid,  therefore  the 
correct  equation  wmuld  be  : — 

AgN03  + HCl  = AgCl  4-  HNO3 

Silver  , Hydrochloric  _ Silver  , Nitric 

nitrate  acid  ~ chloride  acid. 

In  words,  one  molecule  of  silver  nitrate  reacting 
with  one  molecule  of  hydrochloric  acid  produces  a 
molecule  of  silver  chloride  and  a molecule  of 
nitric  acid. 

Reversible  Reactio7is. — If  steam  is  passed  over 
finely-divided  iron  heated  to  redness,  it  undergoes 
decomposition.  Its  hydrogen  is  set  free,  and  the 
oxygen  combines  with  the  iron  to  form  oxide  of 
iron,  as  represented  by  the  following  equation  ; — 

SFe  + 4H2O  = Fe304  + 4H2 

Three  Four  One  mole-  Four 

atoms  of  -f  molecules  = cule  of  oxide  + molecules 

iron  of  water  of  iron  of  hydrogen. 

If  this  oxide  of  iron  is  taken  and  heated  to  red- 
ness in  a current  of  hydrogen  it  produces  metallic 
iron  and  water,  in  accordance  with  the  equation  : 

Fe304  + 4H2  = 3Fe  + 4H.O. 

This  last  equation  is  seen  to  be  the  reverse  of  the 
first  one,  and  the  reaction  is  said  to  be  a reversi- 
ble one.  A reversible  reaction  is  usually  repre- 
sented by  removing  the  sign  of  equality  from  the 


42 


PHOTOGRAPHIC  CHEMISTRY. 


equation  and  introducing  two  arrows  pointing 
in  opposite  directions,  thus  : — 

3Fe  + 4H„0  g ">  FegO^^  + 4H2. 


This  docs  duty  for  the  two  equations  given  above. 

Calculation  of  the  Amount  of  a Compound 
Produced  during  a Chemical  Reaction. — A chemi- 
cal equation  not  only  shows  the  reacting  substances 
and  the  products  obtained,  but  it  also  shows  the 
w’^eights  of  the  various  bodies  taking  part  and 
being  produced  in  the  reaction.  In  preparing 
emulsions  for  dry  plates  or  making  collodion  for 
wet  plates,  silver  chloride,  bromide,  and  iodide 
are  used  in  varying  proportions  to  suit  the 
subject.  Say,  for  example,  it  is  necessary  to  as- 
certain the  amount  of  each  of  these  silver  salts 
obtainable  from  ten  grammes  of  silver  nitrate,  and 
also  the  weight  of  sodium  chloride,  bromide,  and 
iodide  necessary  to  produce  them.  Taking  silver 
chloride,  the  equation  is  first  written  down  : — 
AgN03  -f-  NaCl  - AgCl  + NaN03 


— irn-'7  _L  Diolee.  wt.  _ niolec.  wt.  , molcc.  wt. 
molec.  wt.  - 1G9  7 + ^ - ^,^3.2  + ^55 

From  the  equation  it  is  seen  that  169‘7  parts  of 
silver  nitrate  require  58'5  parts  of  sodium  chloride 
to  produce  143  2 parts  of  silver  chloride.  To  find 
the  weights  of  the  required  substances  for  ten 
grammes  of  silver  nitrate  is  a matter  of  simple 
proportion. 

(1)  I69’7  grs.  of  AgNOs  produce  143'2  grs.  of  AgCl. 


10 


143-2  „ 
169*7 

143*2  X 10 


169-7 


8*4  grs. 
of  silver 
chloride. 


(2)  169*7  grs.  of  AgNO.  require  58*5  grs.  of  NaCI. 


1 


5> 


10 


1) 


5) 


58*5 

169-7  ” 
58-5  X 10 
169*7 


= 3*4  grs. 
of  NaCL 


MEANING  OF  SYMBOLS  AND  EQUATIONS.  43 


In  a similar  manner  the  quantities  of  silver 
bromide  and  iodide,  together  with  the  necessary 
amount  of  sodium  bromide  and  iodide,  can  easily 
be  calculated. 

Water  of  Crystallisation. — In  cases  of  this 
kind,  where  quantities  of  substances  are  calcu- 
lated, it  is  important  to  ascertain  whether  the 
compounds  that  are  taking  part  in  the  reaction 
contain  water  of  crystallisation.  The  need  of 
this  is  seen  in  the  following  example.  A 5 per 
cent,  solution  of  sodium  carbonate  has  to  be  made 
up  from  washing  soda.  Washing  soda  contains 
sodium  carbonate,  which  has  the  formula  NaoCOg. 
But  washing  soda  contains,  as  well,  ten  molecules 
of  water  of  crystallisation,  and  its  complete  mole- 
cular formula  is  Na2C03  + IOH2O.  Now  a 5 per 
cent,  solution  of  sodium  carbonate  means  five 
grammes  of  anhydrous  Na2C03  per  100  parts  of 
solution. 


Nag  = 23 

X 

2 = 

46] 

r molec.  wt.  of 

C = 12 

X 

1 = 

12  1 =106- 

3 anhyd  r 0 u s 

0.,  - 16 

X 

3 = 

48  J 

1 sodium  car- 

10H2O  = (2  16) 

X 

10  = 

180 

(.  bonate 

Molec.  wt.  of  soda  = 286 

For  106  parts  of  anhydrous  NaaCOg  286  parts  of  soda 
„ 1 part  „ 

„ 5 parts  ,, 


From  this  it  is  seen  that  if  the  sodium  carbonate 
is  anhydrous,  that  is,  contains  no  water  of  crystal- 
lisation, all  that  is  necessary  is  to  weigh  out  5 
grammes  and  make  up  to  a volume  of  100.  But 
as  washing  soda  has  to  be  used,  and  this  contains 
water  of  crystallisation,  13*45  grammes  must  be 
laken. 


286  [are  required. 
106 

286  X 5 
106  “ 

ol  soda. 


44 


CHAPTER  IV. 

WATER,  ITS  PROPERTIES  AND  IMPURITIES. 

Physical  Properties. — Water  being  the  chief  sol- 
vent used  for  preparing  photographic  solutions,  a 
brief  consideration  of  its  chemical  and  physical 
properties  should  be  of  service  to  photographers. 
Water  exists  in  the  three  physical  forms  of  matter, 
as  a solid  (ice),  liquid  (water),  and  a gas  (steam). 
Below  0°  C.  it  takes  the  solid  form,  and  at  100° 
C.,  under  a pressure  of  760  mm.  of  mercury,  it 
is  converted  into  the  gaseous  form,  steam.  Water 
at  different  temperatures  undergoes  some  very  re- 
markable changes  in  volume.  If  100  cub.  in.  of  ice 
is  taken  and  gradually  heated  to  0°  C.,  it  liquefies, 
and  the  water  produced  occupies  about  90  cub.  in. 
When  the  temperature  has  risen  to  4°  C.,  the 
water  undergoes  another  contraction  in  volume 
and  now  occupies  about  89  cub.  in.  Above  4°  C. 
the  water  gradually  expands  till  the  temperature 
of  100°  C.  is  reached.  Its  volume  is  now  roughly 
92  cuh.  in.  If  completely  converted  into  steam  it 
increases  in  volume  to  about  160,000  cub.  in. 

Water  as  a Unit  of  Weight. — From  the  above 
it  will  readily  be  observed  that  water  is  at  its 
maximum  density — that  is,  greatest  weight — at  a 
temperature  of  4°  C.  The  weight  of  one  cubic 
centimetre  of  distilled  water,  at  a temperature  of 
4°  C.,  is  the  standard  weight  of  the  metric  system, 
and  this  weight  of  water  is  termed  the  gramme. 

The  Heat  Unit,  or  C alorie.— Owing  to  the  great 
heat  capaeity  of  water  it  is  taken  as  the  heat  unit 
in  comparing  heat  values.  The  unit  of  heat  may 
be  defined  as  the  amount  of  heat  necessary  to  raise 
a unit  weight  of  water  through  a unit  range  of 
temperature.  That  is,  the  amount  of  heat  re- 


WATER,  ITS  PROPERTIES  AND  IMPURITIES.  45 


quired  to  raise  one  gramme  of  water  from  0°  C. 
to  1°  C.  This  heat  unit  is  termed  the  calorie. 

Specific  Gravity,  The  melting  and  boiling 

points  of  water  are  used  for  calibrating  thermo- 
metric scales.  Water  is  also  taken  as  the  stan- 
dard in  the  determination  of  specific  gravities. 
The  specific  gravity  of  a substance  is  the  weight 
of  the  body  divided  by  the  weight  of  an  equal 
volume  of  water. 

Solubility  of  Solids.— extent  to  which 
solid  substances,  under  the  same  conditions,  are 
soluble  in  water,  varies  very  considerably.  For 
instances,  bodies  such  as  chalk,  iron,  and  barium 
sulphate  are  practically  insoluble  in  watery 
calcium  sulphate  and  calcium  hydrate  are 
only  slightly  soluble ; whereas  pyrogallol, 
potassium  carbonate,  and  “ hypo  ’’  are  readily 
soluble.  In  any  case,  however,  there  is  a 
limit  to  the  amount  of  soluble  solid  dissolved 
by  a given  quantity  of  water.  When  the  water 
has  taken  up  as  much  of  the  solid  as  it  can,  we 
have  what  is  termed  a saturated  solution.  The 
amount  of  substance  required  to  produce  a satu- 
rated solution  is,  as  a rule,  greater,  the  higher  the 
temperature  employed,  though  no  simple  relation 
is  observed  between  the  amount  dissolved  and 
the  temperature.  There  are  a few  substances  more 
soluble  in  cold  water  than  hot.  In  the  following 
table  a few  substances  are  given,  together  with 
their  solubility  in  100  parts  of  water  : — 


Suhstance. 

O^C. 

20’C. 

50^0. 

lOO^C. 

Sodium  Chloride  

35-5 

36-0 

37-0 

39-6 

iMercuric  Chloride 

o'7 

7-4 

11-3 

54-0 

Potassium  Nitrate 

13-3 

31-2 

85-0 

246-0 

Water  of  Crystallisation. — Many  metallic  sub- 
stances separate  from  their  aqueous  solutions  in 
crystalline  form,  in  union  with  water.  Sodium 


46 


PHOTOGRAPHIC  CHEMISTRY. 


carbonate  separates  from  its  solutions  in  crystals 
containing  ten  molecules  of  water.  This  com- 
bined water  is  termed  ‘‘  water  of  crystallisation.” 
On  submitting  the  crystals  to  heat,  this  water  is 
driven  off.  In  some  cases  the  water  of  crystallisa- 
tion is  removed  by  merely  exposing  the  crystals 
for  a short  time  to  a dry  atmosphere;  they  are 
then  said  to  effloresce. 

Eain  Water. — Rain  water  is  the  purest  form  of 
natural  water,  and  will  in  nearly  all  cases  be 
quite  as  suitable  for  photographic  purposes  as 
distilled  water,  provided,  of  course,  that  it  is 
collected  in  clean  receptacles.  It  is  rather  inter- 
esting to  notice  that  rain  water  obtained  near 
the  sea,  especially  if  high  winds  have  been  preva- 
lent, usually  contains  sodium  chloride. 

River  Water. — The  composition  of  river  water 
varies  very  considerably,  and  will  depend  upon 
the  strata  over  which  the  water  flows.  For  in- 
stance, the  amount  of  dissolved  solid  matter  in  a 
river  such  as  the  Dee  in  Scotland  is  only  about 
5*6  parts  per  100,000.  But  Thames  water,  having 
for  the  most  part  a drainage  area  of  chalk,  con- 
tains about  30  parts  of  dissolved  matter  per 
100,000  parts  of  water. 

Hard  and  Soft  Water. — Waters  containing 
much  solid  matter  in  solution  form  a lather  with 
soap  only  with  difflculty,  and  there  is  a sense  of 
harshness  when  rubbed  through  the  fingers.  Such 
waters  are  said  to  be  hard.  On  the  other  hand, 
waters  which  lather  readily  with  soap,  and  are 
soft  to  the  touch,  are  termed  soft  waters.  The 
“ hardness  ” of  a water  is  due  to  the  presence 
of  dissolved  calcium  and  magnesium  salts,  princi- 
pally the  carbonates,  sulphates,  and  chlorides. 
If  the  hardness  of  a water  is  due  to  the  occurrence 
of  carbonates,  it  can  be  rendered  soft  by  simply 
boiling  it.  The  chalk  and  magnesium  bicarbon- 
ates undergo  decomposition,  and  the  carbonates 
are  deposited  in  the  boiler,  and  constitute  the 


WATEE,  ITS  PROPERTIES  AND  IMPURITIES.  47 


“ fur.”  Hardness  removed  by  boiling  is  termed 
“ temporary  ” hardness.  The  sulphates  and 
chlorides  of  magnesium  and  calcium  are  not 
removed  by  boiling,  and  hardness  due  to  the 
presence  of  these  salts  is  termed  “ permanent  ” 
hardness. 

Examination  of  Water  for  Photographic  Pur- 
poses.— The  photographer  can  roughly  test  the 
water  he  has  to  use  in  the  following  manner  : — 
Colour:  If  the  water  is  of  a pale  brown  colour 
this  indicates  dissolved  organic  matter,  probably 
of  a “ peaty  ” nature.  This  is  bad,  as  it  is  apt 
to  stain  photographic  papers.  Calcium  and  Mag- 
nesium: A white  precipitate  obtained  by  adding 
a solution  of  ammonium  oxalate  shows  the  pres- 
ence of  calcium  and  magnesium.  These  metals 
will  be  precipitated  as  insoluble  oxalates  when 
making  up  the  ferrous  oxalate  developer  or  any 
other  solution  containing  a soluble  oxalate. 
Sulphates : A white  precipitate,  produced  by  add- 
ing a solution  of  barium  chloride,  insoluble  in 
hydrochloric  acid,  shows  that  sulphates  are 
present  (permanent  hardness).  Chlorides:  If 

silver  nitrate  solution  produces  a white  precipi- 
tate, insoluble  in  dilute  nitric  acid,  this  shows  the 
presence  of  soluble  chlorides,  and  they  will  react 
in  this  manner  to  form  an  insoluble  precipitate 
with  any  bath  containing  silver  nitrate.  Am- 
monia, : A brown  precipitate  is  obtained  by  add- 
ing a small  quantity  of  Nessler’s  reagent.  This 
shows  the  presence  of  ammonia.  If  present  in 
small  quantities,  it  has  very  little  action  on  most 
photographic  solutions,  but  in  large  amounts  it 
precipitates  iron  from  its  solutions.  As  a rule, 
most  of  it  may  be  removed  by  boiling  the  water. 
Nessler’s  reagent  is  prepared  by  adding  a solution 
of  potassium  iodide  to  a solution  of  mercuric 
chloride  till  the  red  precipitate  first  formed  re- 
dissolves. This  clear  solution  is  then  made 
strongly  alkaline  with  caustic  potash. 


48 


CHAPTER  V. 

OXYGEN  AND  HYDROGEN  PHOTOGRAPHICALLY 
CONSIDERED. 

The  apparatus  .required  for  the  experiments 
described  in  this  chapter  is  of  a very  simple  kind, 
and  can  be  bought  for  a few  shillings  from  any 
dealer  in  chemical  accessories.  The  articles  neces- 
sary are  : One  4-oz.  hard  glass  flask  (Fig.  18),  one 
4-oz.  soft  glass  flask  with  wide  neck  (Fig.  19),  one 
thistle  funnel  (Fig.  20),  one  retort  with  stopper 


Fig.  18. — Glass  Heating  Fig.  11>.  — Wide-necked 
Flask.  Glass  Flask. 

(Fig.  21),  ^-Ib.  soft-glass  tubing,  one  dozen  test 
tubes  (Fig.  22),  and  one  set  of  cork  borers.  A 
few  jam  jars,  assorted  corks,  and  other  articles 
will  also  be  necessary,  and  they  will  be  men- 
tioned as  required. 

The  above  apparatus  having  been  provided,  the 
study  of  the  subject  can  be  commenced. 

Bow  Oxygen  is  Obtained. — The  element  oxygen 
is  of  interest  to  the  photographer,  as  it  is  this  gas 
which  causes  his  “ pyro  to  turn  brown  and  his 
sodium  sulphite  to  become  useless.  Also  it  enters 
very  largely  into  a great  many  photographic  re- 


.OXYGEN  AND  HYDROGEN. 


49 


actions.  Oxygen  forms  roughly  one-fifth  by  volume 
of  the  atmosphere,  the  remaining  four-fifths  being 
a very  inactive  gas  called  nitrogen  (together  with 


Fig.  20. — Thistle  Funnel. 


small  quantities  of  argon,  neon,  etc.).  It  is  a 
very  tedious  and  lengthy  process  to  obtain  oxygen 
in  a state  of  purity  from  the  atmosphere.  It  is. 


however,  readily  obtained  by  strongly  heating 
many  substances  rich  in  oxygen,  such  as  potassium 
chlorate  (chlorate  of  potash),  manganese  dioxide 
(pyrolusite),  etc.  Or,  again, 
it  may  be  obtained  by  heat- 
ing certain  oxides  in  which 
the  oxygen  is  combined 
only  very  feebly,  such  as 
mercuric  oxide  or  silver 
oxide. 

Method  of  Pre^mring 
Oxygen. — Into  the  small 
hard-glass  flask  introduce  an  intimate  mixture, 
bf  about  equal  quantities,  of  potassium  chlorate 
and  manganese  dioxide,  so  as  to  about  half  fill  it. 
Next  fit  it  with  a tightly-fitting  cork  and  delivery 
D 


50 


PHOTOGEAPHIC  CHEMISTRY. 


tube,  as  in  Fig.  23.  The  end  of  the  delivery  tube 
dips  beneath  a shallow  tin  can,  having  a hole  cut 
in  the  top  and  another  at  the  bottom,  standing 
in  a basin  of  water.  A small  jam-jar  is  next  filled 
with  water  and  brought,  mouth  downwards,  over 
the  top  of  this  tin.  The  contents  of  the  flask  are 
now  carefully  heated  and  the  evolved  gas  (oxygen) 
collected.  The  first  few  bubbles  of  gas  should  not 
be  allowed  to  enter  the  jar,  as  this  consists,  for 
the  most  part,  of  air.  Collect  four  jars  of  oxygen 
and  cover  their  mouths  with  stout  pieces  of  card- 
board. 


Fig.  23. — Method  of  Preparing  Oxygen. 


Experiments  with  Oxygen. — Experiment  1 : A 
glowing  splint  is  introduced  into  a jar  of  the 
gas;  this,  it  will  be  observed,  instantly  relights. 
A piece  of  wood  charcoal  is  fixed  to  a piece  of 
stout  iron  wire,  heated  to  redness,  and  quickly 
plunged  into  the  jar;  the  charcoal  burns  with 
great  brilliancy,  throwing  off  a large  number  of 
sparks.  Now  pour  a little  clear  limewater  into 
the  jar,  and  notice  that  it  instantly  becomes  of  a 
milky  colour. 

Experiment  2 : Attach  a small  piece  of  stick 
sulphur  to  a piece  of  stout  iron  wire  as  in  Experi- 
ment 1.  Hold  it  in  a flame  till  it  is  well  alight, 
and  then  introduce  it  into  another  jar  of  oxygen. 
Observe  that  the  sulphur  burns  with  a very  feeble 


OXYGEN  AND  HYDROGEN. 


61 


flame  in  ordinary  air,  but  with  a bright  lilac 
flame  in  the  oxygen.  After  the  sulphur  has 
finished  burning,  pour  a little  blue  litmus  solution 
into  the  jar  and  notice  that  the  liquid  turns  red. 

Experiment  3 : Fix  a coil  of  very  thin  iron 
wire  into  a piece  of  cardboard  large  enough  to 
cover  the  top  of  the  jar.  Heat  the  end  of  the  wire 
till  it  is  red-hot,  and  quickly  introduce  into  the 
oxygen.  The  iron  burns  with  great  ease.  Ex- 
amine the  bottom  of  the  jar,  and  notice  the  black 
powder  produced  from  the  burning  iron. 

Experiment  4 : A clear,  strong  solution  of 
freshly  made  pyrogallol  (pyrogallic  acid)  and 
potash  is  added  to  another  jar  of  oxygen  and 
allowed  to  stand  tightly  corked.  In  a compara- 
tively short  time  the  solution  becomes  dark  brown, 
and  if  allowed  to  remain  it  turns  black.  This 
darkening  in  colour  is  due  to  the  fact  that  alka- 
line pyrogallol  is  a powerful  absorber  of  oxygen, 
and  from  this  experiment  we  can  see  how  impor- 
tant it  is  to  keep  developing  solutions  containing 
“ pyro  ” from  the  action  of  the  oxygen  contained 
in  the  air. 

Oxides  and  Oxidation. — We  have  seen  in  the 
first  three  experiments  that  substances  burn  much 
more  brightly  in  oxygen  than  in  air ; owing  to 
this  fact,  oxygen  is  termed  a powerful  supporter 
of  combustion.  When  substances  burn  in  oxygen 
there  is  a chemical  reaction  taking  place— that  is 
the  oxygen,  during  the  combustion,  unites  with 
the  burning  body  to  form  a new  compound,  which 
is  called  an  oxide.  When  carbon  burns  in  oxygen 
a new  body  is  produced,  namely,  oxide  of  carbon, 
which  is  known  as  carbon  dioxide,  or  carbonic 
acid.  Carbon  dioxide  forms  a white  compound 
(chalk)  when  added  to  a solution  of  lime  water, 
hence  this  is  a useful  reagent  for  detecting  the 
presence  of  this  gas.  The  combustion  of  sulphur 
in  oxygen  results  in  the  formation  of  oxide  of 
sulphur,  or,  as  it  is  termed,  sulphur  dioxide. 


52 


PHOTOGRAPHIC  CHEMISTRY. 


This  sulphur  dioxide,  in  the  presence  of  a little 
water,  forms  another  new  compound  known  as 
sulphurous  acid,  and  its  presence  is  indicated  by 
the  change  of  colour  on  the  addition  of  the  blue 
litmus  solution.  Litmus  is  a vegetable  colouring 
matter,  obtained  from  certain  lichens,  and  has  the 
property  of  changing  colour  in  the  presence  of 
acids  and  alkalis.  With  acids  it  is  red,  and  with 
alkalis  blue.  A neutral  solution — that  is,  one  in 
which  neither  acid  nor  alkali  predominates — is  of 
a violet  or  purple  colour.  The  black  compound 
obtained  from  the  burning  iron  is  oxide  of  iron. 
When  substances  unite  with  oxygen  the  process  is 
termed  one  of  oxidation.  By  the  oxidation,  then, 
of  carbon,  sulphur,  and  iron,  carbon  dioxide, 
sulphur  dioxide,  and  black  oxide  of  iron  are  ob- 
tained respectively.  It  has  also  been  noticed  that 
alkaline  pyrogallol  (Experiment  4)  readily  ab- 
sorbs oxygen,  hence  it  is  said  that  pyrogallol  in 
the  presence  of  alkali  (in  this  case,  the  potash)  is 
a substance  which  readily  undergoes  oxidation. 

Photo-oxidation. — The  process  of  oxidation  is, 
in  a large  number  of  instances,  assisted  by  the 
action  of  light,  and  the  operation  is  then  termed 
one  of  photo-oxidation.  This  photo-oxidation 
plays  a very  important  part  in  many  photo- 
graphic operations,  but  it  will  suffice  here  to 
mention  a few  typical  examples.  Dry  potassium 
iodide  is  a stable  substance,  but  moist  potassium 
iodide  undergoes  photo-oxidation  in  the  presence 
of  sunlight,  and  is  converted  into  potassium 
hydrate  and  free  iodine.  Many  coloured  sub- 
stances, such  as  certain  of  the  organic  dyes, 
undergo  photo-oxidation,  with  more  or  less  ease, 
when  exposed  to  the  sunlight  and  atmospheric 
oxygen,  and  are  thereby  converted  into  colourless 
compounds.  The  dark  product  obtained  by  ex- 
posing silver  chloride  to  the  action  of  light  is, 
according  to  some  investigators,  due  to  the  absorp- 
tion of  oxygen. 


OXYGEN  AND  HYDROGEN. 


53 


Preparation  of  Hydrogen. — This  substance  is 
not  of  direct  interest  to  the  photographer ; but  as 
it  enters  indirectly  into  a certain  number  of 
operations,  principally  those  of  reduction,  its 
method  of  preparation  is  here  given.  The  small 
wide-mouthed  flask  is  required,  a cork  pierced 
with  two  holes,  a thistle  funnel,  and  delivery  tube. 
The  apparatus  is  arranged  as  in  Fig.  24,  care 
being  taken  to  see  that  the  thistle  funnel  reaches 
to  the  bottom  of  the  flask.  In  place  of  the  small 
flask  mentioned  above,  a small  bottle,  having  a 
wide  neck,  will  do  equally  well  for  generating 


the  hydrogen,  as  the  apparatus  does  not  require 
the  aid  of  external  heat.  Into  the  flask  or  bottle 
is  placed  a small  quantity  of  granulated  zinc  or 
small  iron  nails.  Dilute  sulphuric  acid  (one  part 
of  acid  to  three  of  water;  note,  always  add  the 
acid  to  the  water,  and  not  vice  versa)  is  next 
poured  down  the  thistle  funnel  equal  in  amount  to 
about  a third  the  capacity  of  the  flask.  The  gas 
should  be  collected  in  test  tubes;  on  no  account 
in  large  jars.  Allow  the  first  few  bubbles  of  gas 
to  escape,  so  that  all  tlie  air  may  be  expelled,  as 
was  done  in  the  case  of  the  oxygen. 

Experiments  with  Experiment  1 : 

Fill  a test  tube  with  the  gas,  cover  the  mouth  of 
the  tube  with  the  thumb,  remove  from  the  basin, 


64 


FHOTOGKAPIIIC  CHtlMISTliY. 


hold  mouth  downwards,  and  introduce  a light.  If 
all  the  air  has  been  driven  out  from  the  generating 
flask  before  collecting  the  hydrogen,  the  gas  will 
extinguish  the  taper,  but  will  burn  itself  with  a 
pale  blue  flame  at  the  mouth  of  the  tube. 

Experiment  2 : A test  tube  is  filled  to  about  a 
third  of  its  capacity  with  water  and  brought  over 
the  issuing  gas.  In  this  manner  a mixture  of 
hydrogen  and  air  is  obtained  (containing,  as  al- 
ready mentioned,  oxygen  and  nitrogen).  On  now 
introducing  a light  into  the  mixed  gases  a sharp 
explosion  takes  place.  This  explosion  is  due  to 
the  fact  that  the  hydrogen  unites  with  the  oxygen 
to  form  oxide  of  hydrogen,  or  water. 

Nascent  Hydrogen. — The  hydrogen  produced  in 
the  immediate  neighbourhood  of  the  zinc  (or  any 
other  metal)  and  dilute  sulphuric  acid  is  in  a 
very  active  chemical  form,  and  will  bring  about 
chemical  changes  much  more  readily  than  hydro- 
gen that  has  once  left  the  generating  solution. 
This  active  form  of  hydrogen  is  spoken  of  as 
“ nascent  hydrogen. 

Experiment  3 : Place  a mixture  of  zinc  and 
dilute  sulphuric  acid  in  a small  flask  or  bottle 
and  add  to  it  a strong  solution  of  ferric  chloride. 
The  deep  yellow  colour  of  ferric  chloride  solution 
is  observed  to  become  paler  and  paler,  till  it 
finally  assumes  a very  weak  green  tint. 

Experiment  4 : Into  .another  bottle  evolving 
hydrogen,  place  a small  quantity  of  silver  chloride 
or  bromide.  These  compounds  can  be  obtained 
by  adding  a solution  of  salt  (sodium  chloride)  or 
sodium  bromide  to  a solution  of  silver  nitrate. 
The  resulting  precipitate  is  well  agitated  with  a 
glass  rod,  and  then  collected  on  a filter  paper. 
In  this  case  it  is  observed  that  the  white  silver 
chloride,  if  this  compound  has  been  taken  for  the 
experiment,  gradually  becomes  grey  in  colour. 
This  grey  compound  is  found  on  examination  to 
be  metallic  silver. 


OXYGEN  AND  HYDROGEN. 


55 


Experiment  5 : Make  a strong  solution  of 
potassium  permanganate  (permanganate  , of 
potash),  and  introduce  some  of  this  into  an  ap- 
paratus evolving  nascent  hydrogen,  as  in  Experi- 
ments 3 and  4.  In  a very  short  time  the  richly- 
coloured  permanganate  solution  becomes  colour- 
less. What  is  the  explanation  of  the  changes 
taking  place  in  these  last  three  experiments  ? 
It  is  as  follows  : The  ferric  chloride,  as  its  name 
implies,  is  a compound  of  iron  and  chlorine.  The 
“ nascent  ” hydrogen  produced  on  the  surface  of 
) the  zinc  attacks  the  ferric  chloride  and  robs  it  of 
some  of  its  chlorine,  and  breaks  it  down  into  a 
compound  of  iron  containing  less  chlorine  than 
the  original  ferric  chloride.  This  new  compound 
is  known  as  ferrous  chloride.  In  the  case  of  the 
silver  chloride  the  nascent  hydrogen  removes  all 
the  chlorine  from  the  silver  chloride,  thus  convert- 
ing it  into  metallic  silver.  Potassium  perman- 
ganate is  a substance  very  rich  in  oxygen,  and  the 
“ nascent  ’’  hydrogen  in  its  immediate  vicinity 
abstracts  this  oxygen  and  breaks  the  salt  down 
into  colourless  compounds. 

Chemical  Reduction. — When  a compound  loses 
oxygen,  or  a halogen  (chlorine,  bromine  Or  iodine) 
so  that  it  contains  an  increased  metallic,  but 
a lowered  non-metallic  composition,  it  is  said  to 
be  reduced.  The  operation  is  termed  one  of  re- 
duction. Consequently  “ nascent/’  hydrogen  is 
termed  a reducing  agent,  because  such  changes 
as  mentioned  above  are  brought  about  by  its 
agency. 

Fhoto-reduction. — Generally  speaking,  it  may 
be  said  that  the  behaviour  of  light  on  compounds 
susceptible  to  its  action  is  one  of  reduction.  On 
this  photo-reduction  depend  some  very  important 
photographic  operations,  such  as  the  preparation 
of  blue  prints,  formation  of  the  latent  image,  etc. 
It  may  be  noted  also  that  hydrogen  assists  the 
photo  decomposition  of  silver  chloride. 


66 


CHAPTER  VI. 

THEORIES  CONCERNING  THE  LATENT  IMAGE. 

Introductory  Remarhs.—l^\iQ  change  which  sub- 
stances undergo  under  the  influence  of  light  may 
be  divided  into  two  kinds  : those  of  a physical  and 
those  of  a chemical  nature.  A few  examples  of 
a change  in  physical  properties  might  be  noted. 
For  instance,  powdered  non-crystalline  selenium 
(an  element  very  similar  to  sulphur)  gradually 
becomes  crystalline  when  exposed  to  light.  Under 
ordinary  conditions,  in  the  dark,  this  crystalline 
variety  of  selenium  is  a very  poor  conductor  of 
electricity,  but  under  the  influence  of  light  it 
becomes  a conductor.  Again,  ordinary  yellow 
phosphorus,  a highly  inflammable  substance,  is 
gradually  converted  by  the  prolonged  action  of 
light  into  red  phosphorus,  having  very  different 
properties  from  the  yellow  variety.  In  these  two 
examples  no  chemical  change  has  taken  place, 
non-crystalline  and  crystalline  selenium  are 
identical  in  chemical  properties,  and  the  same  is 
true  of  the  red  and  yellow  phosphorus.  The 
change  in  physical  properties  can  only  be  ex- 
plained by  assuming  that  the  light  has  influenced 
the  molecular  condition  of  the  substance  on  which 
it  has  acted.  Hence  one  important  action  of 
light  is  to  bring  about  a molecular  change. 

Chemical  Changes  Due  to  Light. — Eder  makes 
the  following  generalisations  : — 

(1.)  All  kinds  of  light  from  the  infra  red  to 
che  ultra  violet  are  capable  of  some  sort  of  photo- 
chemical action. 

(2.)  Photo-chemica]  action  is  only  produced 
by  such  rays  as  the  illuminated  body  absorbs,  so 
that  the  chemical  action  of  light  is  closely  associ- 
ated with  the  optical  absorption. 


THEORIES  CONCERNING  THE  LATENT  IMAGE.  5? 

■L 

(3.)  The  sensitiveness  of  a body  towards  rays 
of  a definite  refrangibility  is  increased  by  the  ad- 
mixture of  other  substances  which  absorb  the 
same  rays. 

(4.)  A substance  is,  as  a rule,  decomposed 
faster  by  a given  colour  when  it  is  mixed  with  a 
body  which  absorbs  one  of  the  products  resulting 
from  the  photo-chemical  decompositions,  such  as 
oxygen  or  the  halogens.  From  the  first  generalisa- 
tion, it  is  evident  that  it  is  not  correct  to  sup- 
pose that  violet  light  alone  is  chemically  active. 
For  instance,  hydrogen  sulphide  solution  is  de- 
composed more  quickly  by  red  light  than  by  the 
violet  rays.  The  yellow  rays  of  sunlight  are  the 
most  active  in  producing  the  photo-decomposition 
of  carbon  dioxide  by  the  green  parts  of  plants. 
Another  point  to  bo  noticed  is  that  light  may 
bring  about  the  union  or  disruption  of  two  or 
more  elements.  A mixture  of  hydrogen  and 
chlorine  exposed  to  light  combines  with  explosive 
violence  to  form  hydrochloric  acid. 

Cl^  + H,  = 2HC1. 

But  this  equation  does  not  correctly  explain  the 
reaction,  because  if  the  two  gases  are  dry  it  is 
extremely  difficult  to  make  them  combine.  Hence 
the  presence  of  moisture  is  essential  to  induce  the 
reaction.  Potassium  iodide  in  the  dry  condition 
is  a stable  substance,  but  if  damp  and  exioosed 
to  light  it  gradually  darkens  and  becomes  alka- 
line. This  change  may  be  represented  as  follows  : 

4KI  -t-  2H,0  4-02  = 4K0H  + 2l,. 

The  action  of  light  is  partly  oxidising  and  partly 
reducing,  according  to  the  nature  of  the  substance 
under  its  influence.  Red  light  acts  mostly  as  an 
oxidising  agent  on  metallic  compounds,  and  violet 
light  as  a reducing  agent. 

Photo-Chemical  Extinction. — Rays  which  have 
passed  through  a medium  sensitive  to  light  are 


68 


PHOTOGRAPHIC  CHEMISTRY. 


weakened  in  their  chemical  activity;  in  fact,  if 
the  medium  is  of  sufficient  thickness,  their  chemi- 
cal activity  may  be  destroyed  entirely.  This 
phenomenon  is  termed  “ photo-chemical  extinc- 
tion,’’ and  is  apparently  of  very  frequent  occur- 
rence. From  this  it  follows  that  light  wffiich  is 
chemically  active  performs  a certain  amount  of 
work. 

Fhoto-Chemistry  of  the  Silver  Compounds. — 
The  action  of  light  on  the  compounds  of  silver 
is  by  far  the  most  important  phenomenon  in 
photography.  AH  the  compounds  of  silver,  both 
organic  and  inorganic,  are  affected  by  light  rays. 
The  first  recorded  observation  of  the  darkening  of 
a silver  salt  by  the  action  of  light  was  apparently 
made  by  Johann  Heinrich  Schulze,  of  Halle,  in 
1727.  Silver  chloride,  when  exposed  to  the  light, 
turns  violet,  and  under  continued  exposure  a 
brownish  violet  colour.  On  prolonged  exposure 
to  light  this  salt  slowly  undergoes  decomposition 
and  evolves  chlorine. 

The  Latent  Image. — There  are  various  views 
put  forward  to  explain  the  change  brought  about 
on  the  surface  of  an  exposed  photographic  plate. 
Such  a plate  shows  absolutely  no  difference  in 
appearance  from  one  which  has  not  received  a 
preliminary  light  treatment.  Nor  is  any  loss  of 
weight  detected  even  by  the  most  sensitive  chemi- 
cal balance ; yet  a change  has  taken  place  under 
the  influence  of  light,  because  a marked  difference 
is  observed  when  the  plate  is  submitted  to  the 
action  of  a developer.  The  imperceptible  yet 
important  change  produced  is  termed  the 
“ latent  ” or  invisible  image.  So  far,  then,  the 
following  facts  are  known  with  certainty  : — 

1.  Light,  in  some  cases,  brings  about  a mole- 
cular change  of  a purely  physical  nature,  and  in 
others  chemical  decomposition. 

2.  Silver  chloride  on  prolonged  treatment  with 
light  (a)  darkens,  and  (6)  decomposes  with  the 


Theories  concerning  the  latent  image,  so 


evolution  of  chlorine.  The  composition  and  for- 
mula of  the  residue  is  not  known.  It  evidently 
contains  a higher  percentage  of  silver  than  the 
normal  haloid. 

3.  The  latent  image  is  {a)  produced  by  a very 
short  exposure;  {h)  is  invisible;  and  (c)  there  is 
no  loss  of  weight. 

The  Theory  of  Suh-Salts. — In  order  to  explain 
the  formation  of  the  latent  image  on  an  exposed 
plate,  various  “ theories  ’’  have  been  proposed 
from  time  to  time.  One  which  has  received  a 
great  amount  of  favour  is  known  as  the  Theory 
of  Sub-Salts.  According  to  this  idea,  when  light 
acts  upon  a compound  of  silver  it  removes  a por- 
tion of  the  non-metallic  element,  and  so  increases 
the  percentage  of  silver,  and  the  new  compound 
produced  is  termed  the  “ sub-salt.”  Thus  the 
action  of  light  on  silver  chloride  would  be  repre- 
sented in  this  manner  : — 


The  greater  the  intensity  of  the  light,  the  more 
is  this  sub-salt  formed ; and  on  this  supposition 
the  latent  image  would  consist  of  layers  of  silver 
sub-chloride  of  varying  thicknesses.  This  sub- 
salt theory  was  originally  proposed  by  Fischer 
in  1814,  and  was  stated  very  clearly  by  Wetzlar 
in  1834.  The  theory  is  in  harmony  with  the 
observed  fact  that  light  liberates  chlorine  from 
silver  chloride.  It  also  explains  why  the 
developer  attacks  the  exposed  portion  of  the 
plate  in  preference  to  the  unexposed  part,  be- 
cause the  sub-chloride  is  already  half  reduced  to 
the  metallic  state.  The  fact  that  no  change  of 
colour  is  observed  on  the  exposed  plate  is  ac- 
counted for,  because  of  the  extremely  minute 
amount  of  sub-salt  produced.  Also,  no  loss  in 
weight  would  be  detected,  because  any  liberated 


+ light  = 2 


Silver  chloride. 


Silver  s?(,&-chloride. 


GO 


MOTOGEAPHlC  CHEMISTRY. 


halogen  is  absorbed  by  the  gelatine  on  the  dry 
plate,  or  by  the  silver  nitrate  on  the  wet  plate. 

Existence  of  Sub-Salts  Doubtful. — The  evidence 
for  and  against  the  idea  that  the  latent  image 
consists  of  silver  sub-haloid  may  now  be  examined. 
According  to  Wetzlar  (Schweig’s  “Jahrbuch,” 
1828),  he  obtained  small  quantities  of  silver  sub- 
chloride by  treating  solutions  of  ferric  and  cupric 
chloride  with  silver  leaf.  In  1839  Wohler  passed  a 
current  of  hydrogen  over  silver  citrate  heated 
to  100°  C.  On  analysing  the  residue,  a compound 
having  the  formula  Ag^O  was  said  to  be  present. 
This  Ag^O  would  be  silver  suboxide  and  would 
correspond  to  the  sub-chloride  AgoCl.  Von  Bibra 
(“  Journal  fiir  Prak.  Chem.’’  [2]  12-55)  treated 
Wohler’s  suboxide  with  strong  hydrochloric  acid 
for  some  time,  and  isolated  a body  having  the 
formula  Ag4Cl3.  If  this  is  the  formula  for  the 
sub-chloride  it  contains  a greater  percentage  of 
silver  than  that  represented  by  AggCl.  Many 
chemists  have  repeated  the  work  of  Wohler  and 
Von  Bibra,  and  have  come  to  the  conclusion  that 
the  existence  of  these  salts  is  extremely  doubtful. 
Giintz  says  that  silver  sub-chloride,  AggCl,  is 
obtained  by  treating  sub-fluoride,  Ag2F,  with 
strong  hydrochloric  acid.  He  obtained  the  sub- 
fluoride by  passing  a powerful  current  of  elec- 
tricity through  a concentrated  solution  of  silver 
fluoride,  using  silver  electrodes.  Up  to  the 
present,  the  formula  of  this  sub-fluoride  has  not 
been  established.  Otto  Vogel  (“  Phot.  Mitt.”  [36] 
334)  attempted  to  isolate  these  silver  sub-haloids 
by  allowing  cuprous  chloride,  bromide,  and  iodide 
to  act  upon  a solution  of  silver  nitrate.  On  a 
cupric  salt  silver  nitrate  acts  as  under  : — 

CuBro  + 2AgNOs  = 2AgBr  + Cu(N03)2. 

With  a cuprous  salt  the  reaction  is  expressed  by 
Vogel  as  : 

CU2CI2  -t  4AgN03  = 2Cu(Ag,N03)2  + Cl,. 


THEORIES  CONCERNING  THE  LATENT  IMAGE.  61 


The  three  sub-haloids  were  stable  in  the  air  and 
only  very  slightly  acted  upon  by  the  light.  The 
analytical  results  agreed  with  the  formulse  Ag4Cl2, 
Ag^Bro,  and  AgJ^.  Vogel  finds  that  mercury  does 
not  extract  silver  from  these  sub-haloids.  Under 
the  microscope  complete  homogeneity  is  observed. 
Nitric  acid  attacks  the  compounds,  dissolving 
silver  and  leaving  the  ordinary  haloids,  AgCl, 
AgBr,  and  Agl.  Vogel  explains  the  fact  that 
silver  chloride  and  bromide  are  not  decomposed  by 
nitric  acid  when  exposed  to  light  (which  should 
be  the  case  if  they  are  converted  into  sub-haloids), 
by  assuming  that  the  sub-haloid  at  once  combines 
with  the  ordinary  haloid  of  silver,  to  form  a 
stable  substance.  Waterhouse  (“  Photo.  Jour.,” 
1900)  and  Emszl  (“  Zeits.  Anorg.  Chem.”)  re- 
peated Vogel’s  experiments,  and  came  to  the  con- 
clusion that  sub-haloids  are  not  produced  in  this 
manner.  From  their  investigations  it  appears 
that  Vogel’s  compounds  consist  of  intimate  mix- 
tures of  finely  divided  silver  and  unaltered  haloid. 
The  latent  image  cannot  consist  of  reduced  silver 
and  unaltered  haloid,  as  some  have  suggested, 
since  silver  chloride  darkens  under  nitric  acid, 
and  on  examining  the  acid  no  appreciable  amount 
of  silver  is  found.  If  the  metal  were  produced 
by  the  light  action,  it  would  be  removed  as  fast  as 
formed,  and  the  acid  would  contain  silver  nitrate. 

2’Ae  ^‘Fhoto  Salts  ” of  Carey  Lea. — By  treating 
ammoniacal  solutions  of  silver  chloride  with  fer- 
rous sulphate,  washing  the  resulting  precipitate, 
and  then  treating  with  hydrochloric  acid,  various 
coloured  compounds  are  obtained,  containing  less 
halogen  than  the  original  chloride.  These  sub- 
stances were  termed  “ photo  salts  ” by  their  dis- 
coverer, Carey  Lea  (“  Amer.  Jour.  Science,”  1887), 
as  he  considered  them  identical  with  the  com- 
pounds produced  by  the  action  of  light  on  the 
silver  haloids.  Their  method  of  formation  sug- 
gests the  possibility  of  sub-salts  existing  in 


62 


PHOTOGRAPHIC  CHEMISTRY. 


admixture  with  the  ordinary  haloid.  So  far  no 
chemical  formula  can  be  assigned  with  certainty 
to  these  “ photo  salts.” 

The  Oxy-Ghloride  Theory. — Many  chemists  who 
have  turned  their  attention  to  photography  hold 
the  view  that  the  balance  of  evidence  is  against 
the  idea  that  the  latent  image  is  due  to  the  forma- 
tion of  sub-salts.  In  their  opinion,  the  latent 
image  consists  of  varying  thicknesses  of  silver 
oxy-chloride,  oxy-bromide,  or  oxy-iodide.  The 
silver  haloid,  according  to  this  view,  in  the  pres- 
ence of  light  or  oxygen,  or  water  vapour,  slowly 
loses  a portion  of  its  halogen  and  absorbs  oxygen, 
to  produce  what  is  termed  an  oxy-haloid.  Dr. 
Hodgkinson  examined  the  darkened  product,  and 
as  the  result  of  his  analysis  gave  it  the  formula 
'Ag40Ck  (“  Chem.  of  Photog.,”  page  56).  Baker 
also  found  that  the  darkened  silver  chloride  con- 
tained oxygen,  and  assigned  to  it  the  formula 
Ag20Cl.  Representing  the  action  of  light  on 
silver  chloride,  according  to  this  view  of  the 
change,  the  equations  would  be  : — 

8AgCl  -f  O.  = 2Ag40CL  + 2CI2 
or  8AgCl  2H2O  = 2Ag40Cl3  -f  4HCI 

Silver  Oxychloriile 
(Hodgkinson). 

4AgCl  -8  0.=  2Ag.0Cl  -8  Cl. 
or  4AgCl  -8  2H2O  = 2AgfiCl  + 2HC1. 

Silver  Oxvehloride 
(Baker). 

Facts  Bearing  on  Oxychloride  Theory. — With 
regard  to  the  formation  of  this  oxychloride  the 
following  facts  are  interesting  : Abney  found  that 
silver  chloride  did  not  darken  in  a vacuum,  even 
after  the  expiration  of  several  months,  provided 
it  was  thoroughly  dry.  From  this  experiment 
it  appears  that  oxygen  and  water  vapour  are 
necessary.  Carey  Lea  mentions  that  the  chloride 
does  not  darken  in  thoroughly  dry  air  or  oxygen. 
This  would  make  it  appear  that  it  is  not  the 


THEOETES  CONCEENING  THE  LATENT  IMAGE.  63 


oxygen  which  causes  the  compound  to  darken, 
but  the  presence  of  moisture.  He  also  states,  how- 
ever, that  silver  chloride  darkens  under  perfectly 
dry  petroleum.  As  no  moisture  or  air  is  present 
in  this  case,  it  is  difficult  to  see  how  any  oxy- 
chloride is  formed.  The  same  may  be  said  of  an 
experiment  of  Baker,  who  found  that  silver 
chloride  darkened  under  dry  benzene,  and  showed 
that  the  dark  substance  was  metallic  silver.  These 
last  two  experiments,  however,  are  not  incom- 
patible with  the  idea  of  sub-salts. 

Molecular  Strain  Theory. — Many  are  of  the 
opinion  that  when  light  acts  upon  a silver  haloid 
to  produce  the  invisible  or  latent  image,  no  chemi- 
cal change  takes  place.  They  do  not  deny  that  silver 
chloride  on  prolonged  treatment  undergoes  chemi- 
cal decomposition ; but  their  contention  is  that 
the  duration  of  the  action  of  the  light  in  an 
ordinary  photographic  exposure  is  insufficient  to 
decompose  the  silver  haloid. _ For  instance,  Chap- 
man Jones  (“  Science  and  Pract.  of  Phot.,’’  1895) 
believes  that  all  the  facts  agree  with  the  sup- 
position that  the  developable  image,  that  is,  latent 
image,  consists  of  particles  of  silver  salt  rendered 
less  stable,  but  not  decomposed.  Again,  Liippo 
Cramer  (“  Brit.  Jour.  Phot.,”  1902)  is  of  the 
opinion  that,  in  the  present  position  of  our  know- 
ledge, there  is  a complete  absence  of  proof  that 
normal  photographic  exposure  produces  any 
chemical  change  in  silver  bromide.  To  account 
for  the  formation  of  the  latent  image,  it  is  sup- 
posed that  when  a silver  haloid  is  affected  by 
light,  an  internal  strain  is  set  up  in  the  mole- 
cules, and  that  the  amount  of  strain  is  propor- 
tional to  the  light  intensity.  A similar  strain 
would  also  be  produced  in  the  sensitiser  present. 
The  effect  of  the  molecular  strain  is  to  render 
the  compound  less  stable,  so  that  in  the  presence 
of  a reducing  agent  (a  developer),  molecules  under 
the  greatest  strain  are  the  first  to  be  sundered. 


64 


PHOTOGRAPHIC  CHEMISTRY 


and  so  produce  metallic  silver.  If  the  action  of 
the  light  is  prolonged,  the  strain  becomes  so  great 
that  the  molecule  is  broken  down  with  the  libera- 
tion of  the  halogen,  and  the  production  of  the 
sub-haloid  or  oxy-chloride,  or  some  other  reduc- 


tion product.  (See  Fig,  25  for  rough  diagram- 
matic idea  of  theory.) 

Relay  86  of  Image. — This  molecular  strain 
theory  also  accounts  for  the  relapse  of  the  latent 
image.  The  recovery  of  all  latent  images  is  only 
a question  of  time.  With  some  substances  there 
is  an  immediate  recovery  as  soon  as  the  light  is 


THEORIES  CONCERNING  THE  LATENT  IMAGE.  65 


removed;  with  others  it  takes  longer,  as  in  the 
Daguerreotype,  where  the  latent  image  disap- 
pears after  the  expiration  of  some  hours.  In  the 
ordinary  photographic  plates  recovery  only  hap- 
pens after  the  lapse  of  several  years.  One  of  the 
chief  functions  of  the  so-called  sensitisers  may  be 
to  prevent  the  self-recovery  of  the  molecule  under 
strain,  and  to  make  the  effect  permanent.  The  fol- 
dowing  interesting  facts  may  be  noted  in  connec- 
tion with  this  idea  of  molecular  strain  : Prof. 
Dewar  (“  Proc.  Roy.  Inst.,”  vol.  13,  p.  695)  found 
that  chemical  activity  gradually  decreased  as  the 
temperature  was  lowered.  At  a temperature  of 
— 180°  C.  a highly  active  substance  like  potassium 
does  not  show  any  appreciable  action  when  im- 
mersed in  liquid  oxygen.  By  reducing  the  tem- 
perature to  — 200°  C.  it  was  found  that  an  or- 
dinary photographic  plate  was  still  fairly  sensi- 
tive to  light.  Now  it  is  rather  difficult  to  see 
why  light  should  produce  a chemical  change,  such 
as  a sub-salt  or  oxy-chloride,  on  the  relatively 
chemically  inactive  silver  haloid,  and  yet  at  a 
temperature  of  twenty  degrees  higher  no  chemical 
change  is  obtained  with  the  highly  chemically 
active  potassium  and  oxygen.  It  seems  more 
reasonable  to  suppose  that  the  light’s  action  was 
a purely  physical  one. 

Further  Experimental  Eesearches.  — Major- 
General  Waterhouse  Proc.  Roy.  Soc.,  1900  ”) 
confirmed  Moser’s  experiments,  that  invisible 
images  are  formed  on  pure  silver  or  copper  plates 
when  exposed  to  the  light.  A plate  of  pure  silver 
was  exposed  under  a masked  pattern  to  the  action 
of  sunlight  for  two  hours.  It  was  then  held  over 
mercury  vapour,  and  a distinct  image  of  the  mask 
I was  obtained.  Copper  behaved  in  a similar  man- 
1 ner.  Consequently,  these  experiments  indicate 
that  most  of  the  phenomena  produced  in  the  ex- 
I posure  of  an  ordinary  photographic  plate,  con- 
) taining  on  its  surface  silver  haloids,  can  be 


66 


PHOTOaHAPHIC  CHEMISTRY. 


observed  upon  a plain  plate  of  silver  exposed 
to  the  light  in  the  air  under  ordinary  conditions. 
A printed-out  image  on  a thin  film  of  silver  was 
treated  with  a solution  of  potassium  cyanide. 
The  unexposed  parts  wrinkled,  but  not  the  ex- 
posed parts.  These  experiments  appear  to  be 
best  explained  by  assuming  that  a physical  and 
not  a chemical  change  has  taken  place  under  the 
light’s  action.  The  uncertainty  with  regard  to 
the  correct  composition  of  the  latent  image  is 
due  to  the  great  experimental  difficulty  met  with, 
in  studying  the  problem,  of  isolating  the  ex- 
tremely minute  amount  of  changed  haloid  from 
the  large  amount  of  unchanged  salt. 

Ripening  of  Emulsio7is. — The  sensitiveness  of 
the  silver  haloids  to  light  depends  very  much  upon 
their  physical  condition.'  When  silver  bromide  is 
precipitated  in  the  collodion  emulsion,  a certain 
amount  of  time  elapses  before  it  reaches  its 
maximum  degree  of  sensitiveness,  and  in  order 
to  attain  this  it  is  allowed  to  stand  for  some 
time;  this  process  is  termed  the  “ripening”  of 
the  emulsion.  The  haloids  in  the  gelatine  emul- 
sion, when  just  precipitated,  are  in  about  the 
same  degree  of  sensitiveness  as  those  present  in 
the  collodion  emulsion.  But  if  the  gelatine  emul- 
sion is  heated  in  a water  bath,  it  then  becomes 
much  more  sensitive  than  the  collodion  emulsion. 
It  has  also  been  found  that  the  emulsion  may  be 
“ ripened  ” by  heating  it  with  ammonia  for  a 
short  time.  All  these  ripening  processes  bring 
about  an  increased  size  of  the  particles  of  silver 
haloid.  Freshly  precipitated  silver  bromide  par- 
ticles were  measured  by  Fder,  and  found  to  be 
from  ‘0008  to  ‘0015  of  a millimetre  in  diameter. 
After  being  “ ripened  ” they  had  increased  to 
'003  and  '004  of  a millimetre  in  diameter.  There 
is,  however,  a limit  to  all  ripening  processes,  as  it 
is  found  that  the  silver  haloids  attack  the  gela- 
tine after  a certain  time,  and  become  partially 


THEORIES  CONCERNING  THE  LATENT  IMAGE.  67 

reduced.  Plates  covered  with  an  over-ripened 
gelatine  emulsion  exhibit  general  fog  when  im- 
mersed in  the  developer. 

Sensitisers. — Another  important  matter  to  be 
considered  in  connection  with  the  action  of  light 
on  the  silver  haloids,  and  other  compounds  sus- 
ceptible to  its  influence,  is  the  function  of  sen- 
sitisers.  It  is  found  that  the  light’s  effect  is 
greatly  accelerated  by  the  presence  of  another 
body  capable  of  absorbing  the  halogens.  Any  sub- 
stance which  behaves  in  this  way,  so  as  To  increase 
sensitiveness,  is  termed  a “ sensitiser.”  In  the 
wet  collodion  process  it  is  the  silver  nitrate  which 
acts  in  this  manner,  and  in  the  gelatine  emulsion 
it  is  the  gelatine. 

Theory  of  Sensitisers. — According  to  the  sub- 
salt and  oxy-haloid  theories,  accounting  for  the 
production  of  the  latent  image,  it  is  supposed  that 
an  infinitesimal  amount  of  halogen  is  liberated 
from  the  silver  haloid.  Now,  on  this  theory  the 
sensitiser  present  absorbs  this  halogen,  and  re- 
moves it  from  the  sphere  of  action  as  fast  as  it 
is  formed.  There  is  a good  deal  of  evidence  for 
this,  from  purely  chemical  reactions,  because  it 
is  found  that  the  velocity  of  a reaction  is  in- 
creased by  removing  the  products  of  the  decom- 
position. But  of  course  this  will  not  account 
for  the  behaviour  of  sensitisers  if  the  light  does 
not  decompose  the  silver  haloid.  As  already  men- 
tioned, on  the  molecular  strain  line  of  reasoning, 
the  supposition  is  put  forward  that  the  action  of 
the  sensitiser  is  to  render  the  molecular  strain 
set  up  in  the  silver  haloid  permanent.  In  other 
words,  the  molecular  strain,  or  stress,  produced 
in  the  sensitiser,  may  retard  the  self-recovery  of 
the  haloid,  or  may  actually  produce,  or  excite, 
a greater  strain  in  the  molecule  of  the  haloid  than 
that  obtained  in  the  absence  of  the  sensitiser. 


( 


C8 


CHAPTER  VII. 

CHEMISTRY  OP  DEVELOPMENT,  TONING,  INTENSIFICA- 
TION, ETC. 

The  Latent  Image  and  Development. — Compounds 
which  render  the  latent  image  visible  are  termed 
developers.  Intimately  bound  up  with  the  pro- 
cess of  development  is  the  constitution  of  this 
latent  image,  and,  as  already  noticed,  the  views 
put  forward  with  regard  to  this  problem  are,  up 
to  the  present,  simply  conjectural.  It  follows 
from  this  that  no  complete  account  of  the 
mechanism  of  development  can  be  given  with  cer- 
tainty. But  though  a connected  chain  of  proof 
cannot  be  put  forward,  many  of  the  accepted 
explanations  are  highly  probable.  The  latent 
image,  according  to  the  various  theories,  consists 
of  layers  of  varying  thickness  of  either  {a)  sub- 
haloid, ijj)  oxyhaloid,  or  (c)  haloid,  under  vary- 
ing degrees  of  molecular  strain. 

Development  by  Mercury  Vapour. — In  the  old 
Daguerreotype  process,  the  latent  image  is  ren- 
dered visible  by  submitting  it  to  the  action  of 
mercury  vapour;  the  amount  of  the  metal  de- 
posited being  proportional  to  the  original  light 
intensity.  The  composition  of  the  compound 
formed  by  the  mercury,  and  that  constituting  the 
latent  image,  whether  subhaloid,  oxyhaloid,  etc., 
is  not  known,  consequently  no  opinion  can  be  put 
forward  to  account  for  the  partiality  of  the  mer- 
cury for  the  latent  image,  in  preference  to  the 
unaltered  haloid. 

Acid  Development  {Ferrous  Sulphate). — This 
developer  is  used  in  the  wet  collodion  and  other 
processes,  in  conjunction  with  acetic  acid.  Fer- 
rous sulphate  is  a very  important  reducing  agent, 
and  in  the  presence  of  substances  rich  in  oxygen, 


CHEMISTKY  OF  DEVELOPMENT,  ETC. 


69 


such  as  silver  nitrate,  it  reduces  them  to  a lower 
state,  being  itself  oxidised  to  ferric  sulphate. 
This  is  readily  shown  by  adding  a small  quantity 
to  a solution  of  silver  nitrate,  a black  precipitate 
of  metallic  silver  being  instantly  produced.  The 
' reaction  may  be  expressed  as  follows  : — 

) 6AgN03  + 6FeSO,  = 2Fe2(SO,)3  + 

Silver  Nitrate.  Ferrous  sulphate.  Ferric  sulphate. 

Feo(N03)e  + 3Ag2. 

Ferric  nitrate.  Silver. 

Now  try  the  effect  of  adding  the  ferrous  sulphate 
to  a small  quantity  of  silver  chloride,  free  from 
silver  nitrate.  Under  ordinary  conditions  no 
reduction  is  observed  in  this  case. 

liestrainers. — On  an  exposed  wet  collodion 
plate  there  are  present  the  latent  image,  un- 
altered haloid,  and  the  sensitiser,  silver  nitrate. 
At  first  sight,  a ferrous  sulphate  developer  would 
appear  to  be  out  of  place  in  view  of  the  above 
reaction,  because  this  silver  nitrate  should  be  in- 
stantly reduced,  all  over  the  plate,  to  the  metallic 
condition,  and  so  cause  general  fog.  Such  would 
be  the  case  in  the  absence  of  the  acetic  acid. 
This  acid  prevents  the  developer  from  acting  too 
rapidly  on  the  silver  nitrate.  Reagents  which 
bring  about  this  retarding  action  are  termed 
restrainers.  Many  substances  behave  in  this 
manner ; for  example,  soluble  organic  acids,  im 
organic  acids,  and  various  viscous  compounds, 
such  as  glycerine,  sugar  solutions,  etc.  ' The  acid 
restrainers  exercise  this  property,  by  virtue  of 
the  fact  that  they  form  stable  compounds  with  the 
silver,  and  so  keep  back,  for  a certain  time,  the 
reducing  action  of  the  developer.  ' Thus  far,  then, 
it  will  have  been  noticed  that  the  developer  re- 
duces the  silver  nitrate,  the  acetic  acid  preventing 
this  reduction  from  becoming  too  rapid,  and  is 
without  action  on  the  unaltered  haloid.  Appar- 
ently the  ferrous  sulphate  does  not  reduce  the 


70 


PHOTOGRAPHIC  CHEMISTRY. 


silver  composing  the  latent  image  (see  Meldola, 
“ Chem.  of  Phot.,”  p.  162).  Where,  then,  does  the 
silver  which  is  deposited  on  the  latent  image  come 
from  ? Evidently,  if  the  silver  of  the  latent 
image,  and  of  the  unaltered  haloid,  does  not 
suffer  reduction,  it  must  be  from  the  sensitiser, 
silver  nitrate,  that  the  image  receives  its  deposit 
of  metal. 

The  Ferrous  Oxalate  Developer. — The  ferrous 
salts  of  certain  organic  acids,  as  would  be  ex- 
pected, can  be  utilised  for  purposes  of  develop- 
ment. One  very  important  salt  of  this  group, 
still  largely  employed,  is  ferrous  oxalate.  This 
must  properly  be  considered  as  an  acid  developer, 
for,  although  it  may  be  used  in  a neutral  con- 
dition, it  works  best  in  an  acid  state,  and  the 
formulae  given  for  this  developer  almost  invariably 
recommend  the  addition  of  an  acid,  generally 
sulphuric,  citric,  or  acetic.  As  ferrous  oxalate 
is  practically  insoluble  in  water,  it  is  brought 
into  solution  in  the  form  of  a double  oxalate  of 
iron  and  potassium.  This  compound  is  readily 
obtained  by  adding  a solution  of  potassium  oxa- 
late to  one  of  ferrous  sulphate  in  the  proportions 
required  by  the  following  equation  : — 

2K,C,0,  -h  FeSO,  = K.FeCCA)^  + K,SO,. 

Potassium  Ferrous  Potassio-ferrous  Potassium 

oxalate.  sulphate.  oxalate.  sulphate. 

The  potassium  sulphate  produced  apparently 
plays  no  part  in  the  development.  The  first 
action  of  the  developer  is  on  the  latent  image, 
which  it  reduces  to  the  metallic  state,  being  itself 
at  the  same  time  oxidised  to  ferric  oxalate.  This 
latent  image  silver,  then,  in  the  presence  of  the 
developer,  decomposes  the  haloid  immediately  be- 
neath it  as  before  described. 

Reactions  of  Ferrous  Oxalate  Developer. — The 
complete  equations  representing  the  reaction  tak- 
ing place  will  necessarily  depend  upon  the  com- 


CHEMISTRY  OF  DEVELOPMENT,  ETC. 


71 


position  of  the  invisible  image.  On  the  sub-salt 
hypothesis  the  first  equation  would  be— 

3Ag,Br  + 3FeC,0,  = Fe,(C,0,)3  + FeBr3 

Silver  sub-  Ferrous  Ferric  oxalate.  Ferric, 

bromide.  oxalate.  bromide. 

+ SAgj. 

Silver  from  latent  image. 

Secondly 

2FeBr3  + 3K,C,04  = Fe,(CA).3  + 6KBr. 
This  latent  image  silver,  %>lus  the  haloid  beneath, 
plus  more  developer,  produces  more  silver  till 
sufficient  density  has  been  obtained  on  the  nega- 
tive. r On  the  oxy-haloid  hypothesis  the  first 
action  of  the  developer  would  be — 


3Ag,OBro 

Silver 

ox  \ bromide. 


+ 6FeC,0,  = 2Fe,(CA)3 

+ 6Ag,. 


-f  2FeBi*3 


The  ferric  bromide  would  then  react  with  the 
potassium  oxalate  as  above.  On  the  molecular 
strain  view  of  the  invisible  image,  the  first  re- 
action would  be — 


6AgBr  -f  eFeC^O,  - 2Fe,(C30j3  -f  2FeBi*3 
3Ag,. 

Action  of  Thiosulphate  in  Ferrous  Oxalate 
Developer. — Although  it  has  been  known  for  a 
considerable  time  that  the  presence  of  a small 
quantity  of  sodium  thiosulphate  increases  the 
activity  of  a ferrous  oxalate  developer,  and  that 
developer  only,  the  mechanism  of  the  reaction 
is  still  very  obscure.  The  thiosulphate  not  only 
assists  development,  but  actually  decreases  the 
time  of  exposure.  According  to  Abney,  the 
period  of  exposure  can  be  reduced  one-third  by 
the  use  of  thiosulphate.  The  favourable  action 
of  this  compound  is  only  noticed  when  it  is  em- 
ployed after  exposure.  If  a plate  is  treated 
to  a preliminary  bath  of  the  thiosulphate  solu- 
tion, then  exposed  and  developed  with  ferrous 
oxalate,  it  shows  general  fog  and  under  exposure. 
Other  compounds  besides  thiosulphate  increase 


n 


tHOTOGEAPHIO  CHEMISTRY. 


the  activity  of  the  ferrous  oxalate  developer. 
Sodium  sulphite,  Na2S03,  is  weaker  in  its 
effect,  and  liver  of  sulphur  stronger,  than  sodium 
thiosulphate.  One  explanation  of  the  action  of 
the  thiosulphate  is  that  it  probably  exercises 
a solvent  effect  on  the  silver  haloid,  and  so  brings 
the  developer  more  into  action.  According  to 
Meldola  (“  Chem.  of  Phot.,’’  p.  188)  the  increased 
activity  of  the  ferrous  oxalate  developer,  in  the 
presence  of  this  compound,  is  due  to  its  reducing 
action  on  the  ferric  oxalate  produced  during 
the  course  of  the  development. 

Alkaline  Development. — The  so-called  organic 
developers,  working  in  alkaline  solution,  form 
another  important  class.  Perhaps  the  most 
generally  useful  of  this  group  is  an  alkaline 
solution  of  pyrogallol,  tri-hydroxy-benzeno, 
which  is  so  largely  used  for  developing  gelatino- 
bromide  or  “ dry  ” plates.  Pyrogallol  not  only 
reduces  silver  nitrate  solutions,  but  also  the 
haloids  of  silver.  In  order  to  moderate  this 
powerful  reducing  action,  restrainers  have  to  be 
used,  and  the  most  suitable  is  found  to  be  a solu- 
ble bromide ; potassium  bromide  being,  as  a rule, 
employed.  The  restraining  action  of  this  salt 
is  probably  due  to  the  formation  of  a stable  com- 
pound with  the  silver  bromide,  which  is  not  so 
readily  acted  upon  by  the  developer.  If  an  ex- 
posed gelatino-bromide  plate  is  treated  with  an 
alkaline  solution  of  potassium  pyrogallate,  a 
negative  is  obtained  in  varying  thicknesses  of 
metallic  silver. 

Origin  of  the  Beduced  Silver. — Attention  must 
be  directed  to  the  source  of  this  silver.  In  the 
dry  plate  no  silver  nitrate  is  present,  as  in  the 
wet  collodion  process ; consequently  the  metal 
must  come  either  from  the  unaltered  haloid  or 
from  the  latent  image.  If  its  source  is  the  un- 
altered haloid,  the  only  manner  in  which  it  can 
be  deposited  on  the  latent  image  is  by  the  former' 


CHEMISTRY  OF  DEVELOPMENT,  ETC. 


73 


dissolving  in  the  alkali  and  then  undergoing 
reduction  by  the  developer,  as  described  in  the 
case  of  silver  nitrate.  If,  however,  unexposed 
silver  bromide  is  repeatedly  washed  with  a soliu 
"tion  of  ammonia,  potash,  or  soda,  of  the  same 
strength  as  that  used  in  development,  it  is  found 
that  only  the  merest  trace  of  the  haloid  is  dis- 
solved, quite  insufficient  to  account  for  the 
density  of  the  silver  deposit  on  the  negative. 
Evidently,  therefore,  this  deposit  must  have  its 
origin  in  the  invisible  image. 

Explanation  of  Density. — It  has  been  proved 
that  the  change  brought  about  by  the  action  of 
light  on  a sensitive  film  is  of  an  extremely  minute 
character,  and  its  equivalent  in  metallic  silver 
would  likewise  be  exceedingly  small.  To  account 
for  the  density  of  a negative,  then,  further  in- 
vestigation has  to  be  made.  Towards  this  end, 
the  following  experiments  appear  to  offer  a clear 
explanation.  Abney  exposed  a dry  plate,  and 
then  covered  half  of  it  with  a collodio-bromid.e 
emulsion.  The  plate  was  next  developed  and  the 
two  films  separated.  On  examining  these  an 
image  was  found  on  each.  The  only  way  of  ac- 
counting for  the  image  on  the  collodion  film, 
which  had  not  been  exposed  to  light,  is  by  as- 
suming that  the  exposed  haloid  on  the  gelatine 
film  is  first  reduced  by  the  developer,  then  this 
liberated  silver,  together  with  more  developer, 
sets  up  a decomposition  of  the  silver  haloid  im- 
mediately above  it.  This  second  layer  of  metal, 
jilus  more  developer,  then  reduces  another  layer 
of  haloid,  and  so  on,  till  no  more  haloid  is  avail- 
able. The  same  action  takes  place  under  the 
reduced  image  on  the  gelatine  plate,  and  extends 
downwards.  In  an  experiment  of  Dr.  Eder  a 
very  thick  gelatino-bromide  emulsion  was  ex- 
posed and  then  developed.  In  this  case  the 

metallic  silver  of  the  image  extended  right 
through  the  thickness  of  the  film,  and  is  clearly 


74 


PHOTOGRAPHIC  CHEMLSTRY. 


formed  from  the  haloid  immediately  below  the 
latent  image. 

Difference  Between  Acid  and  Alkaline 
Development. — Aeid  development,  such  as  that 
described  under  ferrous  sulphate,  differs  in  a 
marked  manner  from  alkaline  development,  es- 
pecially in  the  way  in  which  the  image  on  the 
plate  is  obtained.  In  the  first  method  the  image 
receives  its  silver  from  the  sensitiser,  silver 
nitrate,  which  is  deposited  on  the  plate  and 


grows  upw 
going  dec( 
developme 
the  haloid 

i 

mrd,  without  the  haloid  beneath  under- 
imposition  (see  Fig.  26).  In  alkaline 
nt  no  extra  silver  is  added  to  the  plate, 
is  the  source  of  silver,  and  the  image 

1 .J 

} { 

A 

Fig.  26.-— Acid  Development, 

\ 

1 

mmmm  rnmimm  i 

..,7':  ' A 

Fig.  27. — Alkaline  Development. 

grows  downward.  This  difference  in  the  nature 
of  the  image  is  shown  by  treating  negatives  ob- 
tained by  the  two  methods  with  dilute  nitric  acid. 
The  negative  from  the  acid  developer  is  simply 
restored  to  its  previous  condition,  whilst  that 
from  the  alkaline  developer  has  its  gelatine  sur- 
face pitted  in  places  where  the  image  originally 
rested  (see  Fig.  27). 

Grovjth  of  the  Silver  Image. — It  is  very  re- 
markable that  the  silver  from  the  latent  image, 
under  the  influence  of  the  developer,  should  re- 
duce the  silver  haloid  just  beneath  or  the  silver 
nitrate  above.  It  is  extremely  probable  that  the 
action  referred  to  is  one  of  electrolysis.  In  fact, 
the  experiments  of  Abney  and  Eder  quoted  above 


CHEMISTRY  OF  DEVELOPMENT,  ETC.  T5 

almost  demonstrate  electrolytic  action.  Each 
minute  particle  of  reduced  silver  from  the  latent 
image  can  be  looked  upon  as  constituting  the 
electrodes  of  an  enormous  number  of  minute  cells, 
each  electrolytically  decomposing  the  haloid,  or 
silver  nitrate,  in  its  immediate  neighbourhood, 
and  depositing  the  silver.  Lermontoff’s  experi- 
ment practically  demonstrates  this  electrolytic 
behaviour  of  the  silver.  A solution  of  ferrous  sul- 
phate is  separated  by  a porous  diaphragm  from 
a solution  of  silver  nitrate.  A thin  piece  of 
silver  is  then  bent  so  that  one  end  dips  in  the 
iron  and  the  other  in  the  silver  solution  (Mel- 
dola,  “ Chem.  of  Phot.’’).  In  a very  short  time 
a crystalline  growth  of  silver  makes  its  appear- 
ance on  that  part  of  the  metal  in  the  silver  nitrate. 

Neutral  Development. — Besides  the  two  classes 
of  developers  already  considered — those  which  are 
used  in  an  acid  state,  and  those  employed  with  an 
alkali  or  alkaline  salt — there  is  a third  class  which 
cannot  be  included  under  either  heading.  Ami- 
dol, dianine  (diamido-resorcin  hydrochlorate), 
and  tri-amido-phenol,  for  instance,  may  be  used 
with  sodium  sulphite  as  developers  without  any 
alkali.  Adurol  may  be  used  with  water  only, 
although  it  then  becomes  inconveniently  slow. 
Synthol  can  be  employed,  with  sodium  sulphite, 
either  with  or  without  alkali.  In  addition  to 
these,  there  are  several  other  less  known  develop- 
ing agents  which  are  capable  of  successful  employ- 
ment in  the  absence  of  alkali,  and  must  conse- 
quently be  considered  as  neutral.  The  potassium 
oxalate  solution  employed  for  the  reduction  of  the 
image  in  the  platinotype  process  may  also  be 
regarded  as  a neutral  developer.  It  is  possible 
to  use  eikonogen  without  an  alkali,  but  this  is 
seldom  or  never  done. 

Belapse  and  Destruction  of  Latent  Image. — It 
is  a curious  and  rather  perplexing  fact  that  after 
the  expiration  of  a certain  length  of  time — some 


76 


PHOTOGRAPHIC  CHEMISTRY. 


years  in  the  case  of  a gelatine  film — the  invisible 
image  disappears.  This  phenomenon  is  explained, 
on  the  sub-haloid  and  oxyhaloid  hypotheses,  by 
assuming  that  the  liberated  halogen,  resulting 
from  the  formation  of  these  compounds,  is  ab- 
sorbed by  the  sensitiser,  and  is  then  slowly  re- 
absorbed by  the  latent  image,  in  this  manner 
reverting  to  its  original  state.  Acording  to  the 
molecular  strain  theory,  the  relapse  or  recovery 
of  the  invisible  image  is  of  a purely  physical 
nature;  the  energy  absorbed  by  the  molecules  of 
haloid  and  sensitiser  from  the  original  light 
action  gradually  disappears.  The  molecules  may 
be  compared  to  minute  secondary  batteries,  con- 
sisting of  stored  energy,  which,  when  completely 
run  down,  are  then  in  the  same  condition  as  un- 
exposed molecules  of  the  silver  haloid.  Now  it 
has  been  found  that  not  only  does  the  latent 
image  return  to  its  original  molecular  condition 
by  itself,  but  that  oxidising  agents  and  the  halo- 
gens cause  a like  change. 

Tlalogeji  Absorijtio7i.—OnQ  explanation  of  the 
cause  of  the  destruetive  action  of  these  compounds 
on  the  latent  image  is  that  it  is  due  to  the  gradual 
oxidation  and  re-halogenisation  (or  halogen  ab- 
sorption) of  the  sub-haloid  or  oxy-chloride.  If 
this  is  so,  it  is  simply  a good  illustration  of  a 
reversible  reaction.  From  a chemical  point  of 
view  this  is  an  extremely  probable  explanation, 
if  the  aetion  of  the  light  is  to  decompose  the  silver 
haloid,  because  practically  all  chemical  changes 
under  the  proper  conditions  can  be  reversed. 

Molecular  Disturbance  Theory. — If  the  change 
on  an  exposed  film  is  merely  a case  of  energy  ab- 
sorption for  the  time  being,  this  rehalogenisation 
or  oxidation  hypothesis  is  inadequate.  On  the 
physical  view  of  the  invisible  image,  the  addition 
of  the  destructive  agent  may  result  in  a mole- 
cular disturbance,  or  proceed  further,  and  be 
accompanied  by  a chemical  change  of  the  sensi- 


CHEMISTRY  OF  DEVELOPMENT,  ETC. 


77 


tiser.  For  instance,  in  the  case  of  the  latent 
image  on  a Daguerreotype  plate  this  is  instantly 
destroyed,  if  treated  to  the  vapours  of  a halogen. 
Mn  this  instance  the  excess  of  halogen  simply  dis- 
turbs the  molecular  condition  of  the  altered 
haloid,  and  discharges  its  absorbed  energy,  thereby 
converting  it  to  the  original  haloid.  An  exposed 
gelatino-bromide  plate  loses  its  invisible  image  on 
treatment  with  oxidising  agents  such  as  nitric 
acid,  chromates,  permanganates,  etc.  Now  it  is 
a well-known  fact  that  gelatine  is  susceptible  to 
the  action  of  oxidising  agents,  such  as,  for  ex- 
ample, potassium  bichromate.  Hence  it  is  very 
probable  that  the  oxidising  agent  first  attacks  the 
sensitiser,  that  is,  the  gelatine,  producing  a 
micro-chemical  change,  and  the  molecular  change 
engendered  thereby  sets  up  a corresponding  dis- 
turbance, of  an  opposite  kind  to  that  produced 
by  the  light  originally,  thus  causing  the  altered 
haloid  to  revert  to  its  former  condition. 

Reversal  by  Light  Action. — The  continued  ac- 
tion of  the  light  on  a sensitive  film  also  results 
in  the  partial  or  complete  destruction  of  the 
latent  image.  This  phenomenon  is  known  under 
the  name  of  solarisation  or  reversal.  For  in- 
stance, a greatly  over-exposed  film,  on  develop- 
ment, produces  a positive  instead  of  a negative. 
It  has  been  shown  by  numerous  investigators  that 
the  latent  image  behaves  in  a most  peculiar  man- 
ner under  prolonged  exposure.  Up  to  a certain 
point  it  gradually  gains  in  intensity,  and  then 
slowly  disappears.  It  again  reaches  a certain 
degree  of  intensity,  gradually  diminishing  a 
second  time,  and  so  on.  In  this  connection,  the 
following  facts,  due  for  the  most  part  to  Abney, 
are  interesting  : Solarisation  is  facilitated  by 
a preliminary  exposure  to  diffused  daylight,  by 
the  action  of  powerful  developing  solutions,  and 
by  treating  the  plate  with  a solution  of  some 
oxidising  agent  before  exposure.  According  to 


78 


PHOTOGEAPHIC  CHEMISTRY. 


Abney,  atmospheric  oxidation  is  essential  in  pro- 
ducing solarisation. 

Difficulty  of  Explaining  Reversal. — It  must  be 
confessed  that  it  is  extremely  difficult  to  attempt 
to  explain,  from  either  a chemical  or  physical 
point  of  view,  the  various  facts  underlying  re- 
versal. It  is  very  probable  that  the  prolonged 
exposure  necessary  to  produce  solarisation  is  more 
photo-chemical  in  its  behaviour  than  physical. 
It  is  only  necessary  to  consider  the  possibility  of 
having  on  a plate,  in  less  than  microscopic  .quan- 
tities, unaltered  silver  haloid,  reduced  silver 
haloid  (subhaloid  or  oxyhaloid,  or  a combination 
of  the  two),  silver  haloid  under  molecular  tension, 
gelatine  partly  oxidised,  partly  halogenised,  and 
partly  under  molecular  tension,  to  see  how  very 
complicated  the  subject  becomes.  In  the  present 
state  of  photo-chemical  knowledge  the  so-called  ex- 
planations are  of  a purely  speculative  character. 

Experiment  with  Mercuric  Chloride. — In  con- 
nection with  this  subject  of  the  destruction  of  the 
latent  image,  the  following  experiment  of  Reiss 
(“  Chem.  Zeit.,”  26  [40])O-is  interesting.  He 
utilised  the  well-known  destructive  action  of  mer- 
curic chloride  on  the  invisible  image  to  render 
exposed  plates  fit  for  a second  exposure.  The  ex- 
posed plate,  containing  the  image  to  be  destroyed, 
is  first  treated  with  a solution  of  5 per  cent,  mer- 
curic chloride  and  then  well  washed.  It  is  next 
quickly  immersed  in  an  amidol  developer,  which 
seems  to  facilitate  the  action  of  the  light  in  the 
second  exposure,  dipped  in  water,  and  then  ex- 
posed while  wet.  The  exposure  takes  from  about 
100  to  150  times  that  of  the  first,  and  a much 
longer  development,  to  produce  the  second  latent 
image.  It  is  rather  curious  that  no  fog  results. 
The  negatives  obtained  are  well  covered  in  the 
lights,  but  are  perfectly  clear  in  the  shadows. 
Apparently  the  action  of  the  mercuric  chloride 
on  the  latent  image  induces  a far  greater  change 


CHEMISTRY  OF  DEVELOPMENT,  ETC. 


79 


than  that  involved  in  merely  converting  it  to  its 
original  condition. 

Reduction. — In  some  cases  the  negative,  after 
development,  is  too  dense,  and  takes  a very  long 
^ time  to  print.  This  defect  can  be  remedied  by 
submitting  it  to  reagents  which  will  remove  some 
of  its  silver,  the  process  being  termed  one  of 
photographic  reduction.  Ammonium  persulphate 
is  a substance  capable  of  acting  as  a reducer,  and 
will  presently  be  noticed  in  the  chapter  dealing 
with  sulphur  and  its  compounds. 

Ferric^  Chloride  Reducer. — By  immersing  a 
negative  in  a solution  of  ferric  chloride  the  iron 
is  converted  into  the  ferrous  condition,  and  some 
of  the  silver,  on  the  negative,  into  chloride,  the 
equation  being  as  follows  : — 

2FeCl3  + Ag2  = 2FeCl2  + 2AgCl. 

Ferric  Ferrous 

chloride.  cliloride. 

If  a solvent  for  silver  chloride,  such  as  sodium 
thiosulphate,  be  now  added,  it  dissolves,  and 
thereby  weakens  the  original  deposit.  This 
method  of  reduction  is  not  a good  one  for  two 
reasons.  In  the  first  place  the  operator  is  unable 
to  follow  the  course  of  the  reaction,  as  the  amount 
of  reduction  is  only  observable  after  the  silver 
has  been  removed  by  the  thiosulphate.  Secondly, 
the  ferric  chloride  is  continually  oxidising  the 
thiosulphate,  which  is  also  a disturbing  factor. 

Na^S^Oa  + SFeCla  + 5H.O  = SFeCL  + 2'MaKSO^ 
+ 8HC1. 

EdeFs  Process. — In  Eder’s  process  the  silver 
is  removed  in  the  form  of  oxalate.  A solution  of 
potassio-ferric  oxalate  and  sodium  thiosulphate 
is  added  to  the  negative.  The  metallic  silver  re- 
duces the  ferric  oxalate  to  ferrous  oxalate,  form- 
ing at  the  same  time  silver  oxalate,  which  dis- 
solves at  once  in  the  thiosulphate.  In  this  way 
the  photographer  actually  sees  how  much  silver 


80 


PHOTOGRAPHIC  CHEMISTRY. 


is  being  removed  from  the  negative  undergoing 
reduction  while  the  reaction  is  going  on.  Thus 

(1)  2FeOl3  + + 6KC1. 

Ferric  Potassium  Ferric  Potassium 

cliloride.  oxalate.  oxalate.  chloride. 

(2)  Feo(CoOj3  + Ago  = 2FeC204  + Ag^CaO^. 

Ferrous  Silver 

oxalate.  oxalate. 

(3)  AgoC.04  + 2NaoSo03  = 2AgNaS203  + 

Soluble  silver 
sodium 
thiosulphate 

FTa2C204. 

Sodium  oxalate. 

The  Ferricyanide  Eeducer. — In  Howard 

Farmer’s  process  a freshly-made  solution  of  potas- 
sium ferricyanide  and  sodium  thiosulphate  is  used 
for  reduction.  By  this  method  the  silver  is  slowly 
converted  into  ferrocyanide,  and  it  is  very  prob- 
able that  small  quantities  of  the  ferricyanide  are 
formed  at  the  same  time.  Both  compounds,  how- 
ever, dissolve  at  once  in  the  sodium  thiosulphate, 
thus  allowing  the  amount  of  reduction  to  be  ob- 
served. The  following  reactions  take  place  : — 

(1)  4K3Fe(CN),  -1-  Ag4  = 3K4Fe(CN)3  + 

Potassium  Potassium 

ferricyanide.  ferrocyanide. 

Ag4Fe(CN), 

Silver  ferrocyanide. 

(2)  8K3Fe(CN)e  + 3Ag.  = 6K,Fe(CN)e  F 

2Ag3Fe(CN), 

Silver  ferricyanide. 

(3)  Ag4Fe(CN)e  -f  4Na.S.03  = 4AgNaS.03  + 

Na4Fe(CN), 

Sodium  ferrocyanide. 

(4)  2Ag3Fe(CN)„  + CNa.S.O,  = 6AgNaSA  + 

2Na3Fe(CN)3 

Sodium  ferricyanide. 

Removal  of  Green  Fog. — Henderson  has  shown 
that  green  fog  can  be  removed,  or  an  overdense 
negative  reduced,  by  placing  it  over  a fairly 
strong  solution  of  potassium  cyanide  for  several 


CPIEMISTRY  OF  DEVELOPMENT,  ETC. 


81 


hours.  The  action  in  this  case  is  probably  due 
to  the  carbon  dioxide  decomposing  the  easily 
ionised  cyanide,  thus  liberating  HCN,  which 
forms  easily  soluble  AgH(CN)o. 

Intensification. — Intensification  is,  of  course, 
the  opposite  to  reduction.  Extra  material  is 
added  to  a weak  negative,  in  order  to  increase 
its  density.  In  most  cases  the  silver  image  is 
first  bleached  by  immersion  in  mercuric  chloride 
solution.  This  bleaching  is  due  to  the  formation 
of  a mixture  of  silver  and  mercurous  chloride, 
thus  : — 

Ag3  + 2HgCl3  = 2AgCl  + Hg,Cl3 

]\Iercuric  Mercurons 

chloride.  cliloride. 

As  this  whitened  image  is  hardly  opaque 
enough  for  printing,  it  is  darkened  either  by 
treating  with  ammonia,  ammonium  sulphide, 
sodium  sulphide,  or  with  a ferrous  oxalate  de- 
veloper. 

The  Blachening  Action  of  Ammonia. — A dilute 
solution  of  ammonia  blackens  the  image,  probably 
by  the  formation  of  complicated  mercurous  and 
silver  derivatives  of  ammonium  chloride ; the  two 
chief  compounds  being  NHoHg2Cl  and  NHAg- 
Hg,Cl. 


II 

Hg 

Hg 

1 

Cl-N^H 

1 

Cl-N-Hg 

1 

Cl— N— Hg 

1 

H 

Vh 

H 

H 

Am  inonium 

Di  mercurous 

Dimercurous 

chloride. 

ammonium 

silver  ammouiuna 

chloride. 

chloride. 

From  some  recent  investigations  of  M.  F. 
Leteur  the  black  compound,  on  analysis,  contains 
silver,  and  these  experiments  confirm  the  formula 
NHAgHg2Cl  proposed  by  Chapman  Jones.  The 
equation  showing  the  change  would  then  be — 

F 


82 


PHOTOGEAPHIC  CHEMISTRY. 


AgCl  + Hg,Cl,  + 3NH3  = NHAgHg^Cl  + 
2NH,C1 

Diinerciiroiis 
silver  ammonium 
chloride. 

In  the  case  of  using  ammonium  sulphide  the 
metals  forming  the  bleached  image  are  converted 
into  sulphides. 

2AgCl  + 2Hg2Cl2  + 3(NH4)2S  = AgaS  + 
2Hg2S  + 6NH,C1 

Intensification  with  Sodium  Sulphite. — Inten- 
sification by  means  of  sodium  sulphite  results  in 
the  partial  dissolving  of  the  chlorides  on  the  film 
as  complicated  sulphites,  the  rest  being  reduced, 
for  the  most  part,  to  the  metallic  state. 

4AgCl  + 2Hg2Cl2  + TNaoSOa  = Ag,  + Hg  + 
Ag2S03  + 3HgNa2  (803)2  + 8NaCl. 
According  to  G.  Hauberrisser  (“  Phot.  Runds- 
chau,’’ 1902)  sodium  thiosulphate  solutions  remove 
from  the  intensified  negative  silver  and  mer- 
cury, leaving  a black  residue.  On  treatment  with 
acid,  large  amounts  of  sulphuretted  hydrogen 
were  produced.  He  concludes  by  saying  that  the 
negative  intensified  by  sulphite  and  mercuric 
chloride  probably  consists  of  mercury  and  silver 
in  union  with  a small  quantity  of  sulphur.  But 
according  to  E.  Valenta  (“  Phot.  Corr.,”  1902)  the 
blackened  image  contains  no  compound  of  silver 
or  mercury  in  combination  with  sulphur.  If  a 
sufficient  amount  of  sulphite  is  used,  it  consists 
entirely  of  metallic  silver  and  mercury.  With  a 
weak  solution  of  sulphite,  or  if  a concentrated 
solution  of  sulphite  is  used  for  a short  time,  the 
blackened  image  consists  of  varying  amounts  of 
silver  chloride,  plus  metallic  silver  and  mercury. 

Intensification  with  Ferrous  Oxalate. — Accord- 
ing to  Chapman  Jones  (“  Roy.  Phot.  Soc.  Jour.,” 
1897)  the  most  reliable  method  of  intensification 
is  by  the  use  of  the  ferrous  oxalate  developer  on 
the  bleached  image.  The  developer  reduces  the 


CHEMISTRY  OF  DEVELOPMENT,  ETC.  83 

chlorides  to  the  metallic  state,  the  increased  den- 
sity being  due  to  the  deposition  of  mercury. 

(1)  Hg.Cl^  + 2FeC.O,  + K,C.O,  = Hg^  + 

Fe,(CA)a  + 2KC1. 

(2)  2AgCl  + 2FeCo04  + K.CoO,,  = Ag^  + 

Fe2(C20,)3  + 2KC1. 

If  the  negative  has  not  sufficiently  gained  in 
intensity,  it  can  be  put  through  the  bleaching  and 
developing  process  again,  so  as  to  deposit  more 
mercury  on  the  film.  This  metal  apparently 
forms  a stable  amalgam  with  the  silver,  owing  to 
the  protective  action  of  the  gelatine. 

Lead  and  Uranium  Intensifiers. — Another 
method  of  intensification  consists  in  the  use  of 
certain  metallic  ferricyanides,  the  more  common 
being  those  of  lead  and  uranium.  The  principle 
of  these  intensifiers  is  the  formation  of  an  insolu- 
ble ferrocyanide  of  the  metal,  by  the  reducing 
action  of  the  silver  on  its  ferricyanide.  If  the 
negative  is  treated  with  lead  ferricyanide,  obtained 
by  adding  lead  nitrate  to  potassium  ferricyanide, 
it  becomes  coated  with  a greyish  deposit  consisting 
of  silver  and  lead  ferrocyanides.  The  changes 
taking  place  may  be  represented  as  follows  : — 

(1)  2K.3Fe(CN),  + = 

Potassium  Lead 

ferricyanide.  nitrate. 

Pb,[Fe(CN)J,  + 6KNO3 

Lead  Potassium 

ferricyanide.  nitrate. 

(2)  2Ag,  + 2Pb[Fe(CN),],  = 
Ag,Fe(CN),  + SPbjFeCCNie 

Silver  Lead 

ferrocyanide.  ferrocyanide. 

The  mixture  of  ferrocyanides  is  then  treated 
with  ammonium  sulphide  or  potassium  chromate, 
forming  the  sulphides  and  chromates  of  silver  and 


84 


PHOTOGEAPHIC  CHEMISTRY. 


lead  respectively,  in  order  to  render  the  negative 
more  suitable  for  printing  purposes. 

(1) 

Ag,Fe(CN),  + Pb,Fe(CN),  + 4(NH,),S  = 
2Ag,S  + 2PbS  + 2(NHJ,Fe(CN)« 

Silver  Lead  Ammonium 

Suli)liide.  Sulphide.  ferrocyanide. 

(2) 

Ag.Fe(CN),  + Pb,Fe(CN)„  + 2K,CrO.  = 

Potassium 

chromate. 

2Ag2CrO,  + PbCrO,  + 2K,Fe(CN)e. 

Silver  Lead  Potassium 

chromate.  chromate.  ferrocyanide. 

In  the  case  of  the  uranium  intensifier,  the  mix- 
ture of  silver  and  uranium  ferrocyanides,  being 
dark  brown  in  colour,  is  sufficiently  opaque.  A 
probable  equation  to  express  the  change  is  as 
follows  : — 

2Ag,  + 2(UO,)3[Fe(CN)J,  = Ag,Fe(CN),  + 

Uranium 
ferricyanide.  (?) 

3(UO,)3Fe(CN), 

Uranium 
ferrocyanide.  (?) 

Album, enised  Paper. — This  is  prepared  by  coat- 
ing paper  with  albumen  containing  ammonium 
chloride.  The  salted  paper  is  sensitised  as  re- 
quired by  floating  on  a solution  of  silver  nitrate.  * 
Evidently  the  paper  now  contains  silver  chloride, 
intimately  mixed  with  the  albumen,  due  to  the 
interaction  of  the  ammonium  chloride  and  silver 
nitrate  : 

AgN03  + NH,C1  = AgCl  + NH,N03 
Another  reaction  also  takes  place  between  the 
albumen  and  the  silver  nitrate,  to  produce  an 
insoluble  compound  of  silver  and  albumen,  whose 
nature  is  not  known.  This  is  usually  termed 
silver  albuminate.’^  That  this  is  the  case  is 
easily  shown  by  adding  silver  nitrate  solution  to 
a solution  of  albumen  in  water.  The  compound 


) 

CHEMISTRY  OF  DEVELOPMENT,  ETC.  85 

is  thrown  down  as  a white  curdy  precipitate. 
The  sensitive  surface  of  albumen  paper  consists  of 
silver  chloride  and  silver  albuminate,  together 
with  a small  quantity  of  silver  nitrate.  In  the 
ready-sensitised  paper  citric  acid  is  present  as 
well. 

Action  of  TAglit  on  Alhumenised  Taper. — Now 
this  is  a truly  formidable  list  of  compounds  to 
have  together,  and  in  the  present  state  of  know- 
ledge very  little  is  known  of  the  changes  brought 
about  in  them  by  the  action  of  light.  In  the  first 
place,  the  constitution  of  albumen  is  unknown, 
and  it  is  very  questionable  whether  the  formula 
given  in  the  text  books  is  true.  Secondly,  the 
composition  of  the  albuminate  is  unknown,  and 
it  is  idle  to  speculate  about  its  photo-decomposi- 
tion. Thirdly,  the  light  decomposition  of  silver 
chloride  is  extremely  vague,  especially  in  the 
presence  of  organic  matter.  The  reddish-brown 
reduction  compounds,  produced  by  the  light’s 
action  on  the  albumen  paper,  may  be  identical 
with  the  photo-salts  of  Carey  Lea  (see  p.  61). 
They  would  thus  consist  of  a series  of  reduced 
compounds,  from  metallic  silver  to  the  unaltered 
product.  Perhaps  the  first  action  of  the  light 
is  to  set  up  an  internal  strain  on  the  molecules 
composing  the  sensitive  surface. 

Action  of  Light  on  Collodio-  and  Gelatino- 
cliloride  Papers. — In  the  “ printing-out  ” papers, 
collodio  and  gelatino  emulsions  of  silver  chloride 
and  citrate  are  used.  The  action  of  light  on 
these,  as  in  the  case  of  albumen  paper,  is  allowed 
to  continue  to  the  period  of  visible  decomposition. 
The  remarks  made  above,  in  connection  with  the 
chemical  changes  taking  place  when  printing  with 
albumen  paper,  apply  also  to  these  various  print- 
ing-out papers ; that  is  to  say,  practically  nothing 
is  known. 

Chemistry  of  Bromide  Printing. — Bromide 
paper  is  a paper  covered  with  a gelatino-bromide 


86 


PHOTOaUAPHlC  CHEMISTRY. 


of  silver  emulsion,  similar  to  that  used  for  coat- 
ing dry  plates,  but  much  slower.  In  this  case  an 
exposure  is  made  behind  the  negative,  but  the 
image  is  invisible.  This  is  then  developed  with 
hydroquinone,  ferrous  oxalate,  or  almost  any 
other  developer.  The  chemistry  underlying  the 
process  is  identical  with  ordinary  development, 
and  it  follows  that  the  picture  is  composed  of 
metallic  silver. 

Printing  in  Platinum. — Platinum  prints  are 
not  secured  by  the  direct  photo-reduction  of  plat- 
inum salts.  Instead,  they  are  obtained  indirectly 
through  the  photo-reduction  of  ferric  oxalate.  It 
has  been  noticed  already  that  ferric  oxalate,  in 
the  presence  of  light,  is  converted  into  ferrous 
oxalate.  In  the  preparation  of  “ blue  ” prints, 
this  ferrous  compound  is  treated  with  potassium 
ferricyanide,  thus  producing  Turnbull’s  blue.  In 
the  platinotype  process, the  ferrous  oxalate  ic 
made  to  reduce  a platinum  salt  to  the  metallic 
state.  In  the  actual  process  a paper  is  covered 
with  ferric  oxalate  and  potassium  chloroplatinite. 
It  is  then  exposed,  whereby  the  ferric  compound  is 
reduced  to  ferrous  oxalate,  the  platinum  com- 
pound remaining  unaltered.  It  is  next  treated 
with  a warm  solution  of  potassium  oxalate.  As 
soon  as  the  ferrous  oxalate  dissolves  in  the  potas- 
sium oxalate,  it  reacts  with  the  potassium  chloro- 
platinite, and  reduces  it  to  metallic  platinum. 
The  reaction  taking  place  may  be  written  : — 


SK^PtCl, 

Polassiimi 

chloroplatinite. 


eFeC.O^  = 3Pt  -h  2Feo(CoOj3 
2FeCL  -h  6KC1. 


+ 


By  sensitising  the  platinum  paper  with  mercuric 
citrate,  obtained  by  adding  mercuric  oxide  to 
citric  acid,  tones  varying  from  yellow,  black, 
brown,  to  red-brown  are  obtained  with  a cold 
developer  (Hiibl,  “ Chem.  Zeit.,”  25). 

Toning. — To  remove  the  objectionable  red- 
brown  colour  of  the  freshly  printed  albumen  or 


Chemistry  of  development,  etc. 


87 


gelatino-chloride  paper,  it  undergoes  the  opera- 
tion of  toning.  As  a rule,  this  consists  of  the 
deposition  of  some  metal  on  the  silver  and  reduc- 
tion products  obtained  in  the  printing. 

Gold  Toning. — In  gold  toning,  a solution  of 
gold  chloride,  AuCla,  or  sodium  chlor-aurate, 
N'aAuCl4,  is  used  in  a neutral  solution,  or  in  the 
presence  of  some  mild  organic  acid.  The  sub- 
stances usually  employed  are  ammonium  sulpho- 
cyanide,  sodium  carbonate,  acetate,  borate 
(borax),  phosphate  or  tungstate.  The  silver  and 
the  brown-red  reduction  products  on  the  paper 
first  of  all  reduce  the  auric  chloride  to  aurous 
chloride,  and  then  this  compound  to  the  metallic 
state.  The  colour  of  the  precipitated  gold  de- 
pends upon  the  rate  of  deposition,  the  strength 
of  the  toning  bath,  and  the  temperature  of  the 
solutions.  The  operation  of  toning  is  probably 
electrolytic  in  its  action,  if  much  silver  is  present 
in  the  print. 

(1)  AUCI3  4-  Ag,  - AuCl  + 2AgCl. 

Auric  Aurous 

chloride.  chloride. 

(2)  2A11CI  + Aga  = AUo  + 2AgCl. 

In  the  fixing  bath  the  thiosulphate  removes  this 
silver  chloride  and  any  not  affected  by  the  light 
originally.  The  precise  action  of  the  thiosulphate 
is  explained  in  Chapter  X. 

Toning  with  Platinum  and  Lead.—\n  platinum 
and  lead  toning  an  exactly  similar  set  of  reactions 
are  produced  as  in  the  case  of  toning  with  gold. 
For  instance,  the  platinum  is  reduced  from  the 
platinic  to  the  platinous  state,  and  then  deposited 
as  the  metal 


PtCl,. 

Platinic 

chloride. 


2PtCl,  + 2Ag,  = Pt,  + 4AgCl. 

Platinous 

chloride. 

or 


2KjPtCl.  + 2Ag,  = Ptj  + 4Ag01  + 4KC1. 


88 


PHOTOGRAPHIC  CHEMISTRY. 


In  the  case  of  a lead  compound,  metallic  lead 
takes  the  place  of  the  silver. 

Uranium  Toning. — Bromide  prints  are  some- 
times toned  with  uranium  compounds.  A choco- 
late deposit  is  produced,  consisting  of  silver  and 
uranium  ferrocyanides.  From  what  has  been 
said  already,  many  other  processes  could  also  be 
used  for  changing  the  colour  of  a bromide  print. 
Gaedecke  considers  that  the  permanence  of 
bromide  prints,  toned  by  the  formation  of  certain 
metallic  ferrocyanides,  is  of  a very  doubtful 
nature.  The  red  and  blue  tones  obtained  by 
uranium  nitrate  and  ferric  oxalate  respectively 
are  not  to  be  trusted  as  regards  permanency.  For 
further  details  concerning  uranium  toning  see 
p.  130. 


89 


CHAPTER  VIII. 

NITEOGEN  COMPOUNDS  EMPLOYED  IN  PHOTOGRAPHY. 

Simple  Compounds  Containing  Nitrogen. — It  is 
proposed  to  consider  in  this  chapter  a few  common 
compounds  containing  the  element  nitrogen,  some 
of  which  are  of  great  use  in  photography.  The 
compounds  to  be  dealt  with  are  enumerated 
below  : — • 

N Nitrogen.  N3O  Nitrous  oxide-  NHo  Ammonia. 

NO  Nitric  oxide.  HNOo  Nitric  Acid. 

Nitrogen. — This  element  occurs  in  a free  state 
in  the  atmosphere,  along  with  oxygen  and  small 
quantities  of  argon  and  neon.  These  last  two 
gases  are  very  similar  in  their  properties  to 
nitrogen.  Nitrogen  is  readily  obtained  by  care- 
fully heating  a solution,  in  water,  of  potassium 
nitrite  and  ammonium  chloride.  It  is  produced 
in  accordance  with  the  following  equations  : — 


(1)  KNO3  + NH,C1 

- NH.NO^ 

-f  KCL 

Potassium  Ammonium 

_ Ammonium 

, Potassiiim 

nitrite.  chloride. 

~ nitrite. 

chloride. 

(2)  NH,NO„  = 

N3  -P 

2H3O 

Ammonium  nitrite  = 

Nitrogen  -i- 

Water. 

Nitrogen  is  characterised  by  its  great  inactivity 
towards  chemical  reagents.  It  has  no  action  on 
blue  or  red  litmus,  is  a non-supporter  of  combus- 
tion, is  non-combustible,  and  has  neither  colour 
nor  odour. 

Oxides  of  Nitrogen. — The  compounds  nitric 
and  nitrous  oxides  are  produced  when  metals  are 
dissolved  in  nitric  acid ; consequently,  they  are 
formed  when  silver  is  treated  with  this  acid,  in 


90 


PHOTOGRAPHIC  CHEMISTRY. 


order  to  convert  it  into  silver  nitrate.  The 
amount  of  each  oxide  of  nitrogen  produced,  how- 
ever, varies  with  the  metal  undergoing  solution, 
and  the  strength  of  the  nitric  acid  used. 

Nitric  Oxide. — This  oxide  is  best  obtained  by 
allowing  dilute  nitric  acid  to  act  upon  metallic 
copper.  The  apparatus  to  be  used  is  illustrated 
by  Fig.  24  (p.  53). 

The  equation  representing  the  change  is  as 
follows  : — • 

3Cu  -!-  8HNO3  = 3Cu(N03)3  + 2H,0  + 2NO 

Copper.  Nitric  acid.  Copper  nitrate.  Water.  Nitric 

oxide. 

If  the  copper  is  replaced  by  metallic  silver  a simi- 
lar change  takes  place,  only,  of  course,  with  the 
formation  of  silver  nitrate  in  place  of  the  copper 
nitrate. 

3Ag  + 4HNO2  = 3AgN03  + 2H2O  + NO 

Silver  nitrate. 

Collect  a jar  of  the  gas,  and  notice  that  it  is 
colourless.  Cautiously  remove  the  cover  of  the 
jar,  and  observe  that  as  soon  as  the  air  is  intro- 
duced it  turns  a fine  brown  colour.  This  brown 
gas  is  due  to  the  formation  of  another  oxide  of 
nitrogen,  known  as  nitrogen  dioxide  or  nitrogen 
peroxide. 

2NO  + 20  = N,0, 

Nitric  oxide  - — {>  Nitrogen  peroxide 

(colourless).  (brown  gas). 

Nitrous  Oxide. — This  oxide  is  produced,  along 
with  other  oxides  of  nitrogen,  when  zinc  is  dis- 
solved in  dilute  nitric  acid.  It  is  best  obtained 
by  heating  ammonium  nitrate.  A quantity  of 
solid  ammonium  nitrate  is  introduced  into  the 
hard-glass  flask  so  as  to  half  fill  it.  The  flask 
is  then  fitted  with  a cork  and  delivery  tube,  and 
arranged  as  in  Fig.  23,  p.  50.  The  ammonium 
nitrate  is  then  cautiously  heated  and  the  gas 


NITROGEN  COMPOUNDS  EMPLOYED. 


91 


collected  over  water.  The  production  of  the  gas 
is  thus  represented  symbolically  : — 

NH4NO3  = N,0  + H,0 

Ammonium  nitrate.  J>  Nitroi<s  oxide. 

Introduce  a glowing  taper  into  a jar  of  the 
gas  and  notice  that  it  instantly  relights.  Hence 
this  gas  is  a supporter  of  combustion.  Allow 
the  contents  of  another  jar  to  mix  with  the  sur- 
rounding air.  No  brown  fumes  are  produced 
in  this  case.  Nitrous  oxide  when  inhaled  has  an 
exhilarating  effect  and  produces  a kind  of  in- 
toxication, and  for  this  reason  it  is  termed 
laughing  gas. 

Connection  with  Photograi)hy. — These  oxides 
of  nitrogen  have  no  direct  interest  in  photo- 
graphic work,  but  in  all  those  cases  where  nitric 
acid  undergoes  reduction,  either  by  the  action  of 
organic  matter  or  of  light,  varying  quantities 
of  nitric  oxide,  nitrous  oxide,  and  nitrogen  per- 
oxide are  obtained. 

Nitric  Acid. — This  acid  is  chiefly  used  in 
photographic  work  for  the  preparation  of 
nitrates,  especially  silver  nitrate,  and  for  the 
production  of  collodion  pyroxylin.  It  is  also 
used  in  conjunction  with  hydrochloric  acid  for 
the  preparation  of  gold  and  platinic  chlorides. 
The  acid  may  be  prepared  in  the  following  man- 
ner : The  small  stoppered  retort  is  taken  and 
filled  to  about  a third  of  its  capacity  with  any 
common  nitrate  that  the  photographer  has  at  his 
disposal.  The  retort  is  then  connected  with  a 
small  flask  resting  in  a basin  of  cold  water  (see 
Fig.  28).  A sufficient  quantity  of  strong  sul- 
phuric acid  is  now  poured  over  the  nitrate  so  as 
to  just  cover  it.  The  contents  of  the  retort  are 
then  cautiously  heated.  In  a very  short  time  the 
sulphuric  acid  attacks  the  nitrate,  brown  fumes 
(principally  nitrogen  peroxide,  due  to  the  occur- 
rence of  bye  reactions)  arc  evolved,  and  a liquid 


92 


PHOTOGHAPHIC  CHEMISTHY. 


is  seen  to  condense  on  the  cooler  part  of  the  re- 
tort. This  liquid  is  the  nitric  acid. 

The  equation  representing  the  change  is  as 
follows,  supposing  potassium  nitrate  to  have 
been  used  : — 


K 


NO,  + H 


H 


> 


SO. 


Totassium  , Sulphuric 
nitrate.  acid. 


K 


^>SO.  + HNO3 

, *■.  Potassium 

_ wii.  hydrogen  + Nitric 

sulphate,  acid. 


The  acid  in  the  receiver,  which  is  in  a very  con- 
centrated form  if  the  materials  in  the  retort  were 
dry,  is  divided  into  two  parts.  To  one  part  is 
added  twice  its  bulk  of  water. 

Experiments  v'ith  Nitric  Acid.~{\)  Into  a 
portion  of  the  strong  acid  introduce  a piece  of 
metallic  lead.  Notice  that  the  lead  is  instantly 
attacked,  but  soon  becomes  covered  with  a white 
compound,  and  the  action  ceases. 


NITKOGEN  COMPOUNDS  EMPLOYED. 


93 


(2)  Introduce  a piece  of  lead  into  a small 
quantity  of  the  diluted  acid.  Observe  that  the 
lead  is  soon  attacked  by  the  acid,  and  if  left 
for  a short  time  completely  disappears.  There  is 
evidently  here  a marked  difference  in  the  be- 
haviour of  metals  towards  the  dilute  and  concen- 
trated acids.  In  the  strong  acid  the  lead  is  con- 
verted into  lead  nitrate,  and  this  forms  a 
protective  covering  over  the  surface  of  the  metal, 
so  that  the  action  soon  ceases.  In  the  case  of  the 
dilute  acid,  lead  nitrate  is  also  produced,  but  as 
fast  as  it  forms  it  is  dissolved  by  the  water,  so 
that  the  surface  of  the  lead  is  continually  kept 
clean,  and  of  course  is  attacked  by  the  acid  as 
long  as  any  remains.  This  is  a very  important 
point  to  notice  when  dissolving  metals  in  acids, 
that  water  must  be  present  in  sufficient  quantity 
to  dissolve  the  salt  produced. 

(3)  Pour  a few  drops  of  the  concentrated  acid 
on  a piece  of  white  paper  or  wood.  Observe  the 
yellow  stain  produced,  which  is  not  removable 
by  washing  with  water. 

(4)  To  a portion  of  the  dilute  acid  add  some 
blue  litmus  solution.  Observe*  that  it  turns  red. 
This  is  an  important  test  for  acids. 

(5)  Add  a small  quantity  of  washing  soda  to 
another  portion  of  the  dilute  acid.  Notice  that 
a brisk  effervescence  occurs.  This  is  due  to  the 
liberation  of  carbon  dioxide. 

(6)  Introduce  a small  quantity  of  the  dilute 
acid  into  a cool  solution  of  ferrous  sulphate. 
Notice  the  ferrous  sulphate  turns  brown.  This  is 
another  important  test  for  nitric  acid. 

Precautions  in  Preparing  Nitric  Acid. — (1) 
After  removing  the  receiver  from  the  basin  of 
water,  be  careful  to  see  that  the  end  of  the  retort 
does  not  dip  beneath  the  surface  of  the  water. 

(2)  To  clean  the  retort,  allow  the  contents  to 
cool  somewhat  and  then  carefully  pour  in  a very 
thin  stream  into  water. 


94 


PHOTOGRAPHIC  CHEMISTRY. 


(3)  Take  great  care  that  the  acid  does  not  get 
on  the  hands,  as  the  skin  is  stained  yellow.  With 
large  quantities  bad  wounds  are  produced. 

Impurities  in  Commercial  Nitric  Acid. — The 
impurities  usually  met  with  are  sulphuric  acid 
and  hydrochloric  acid.  The  last  acid  is  the  worst 
offender,  as  it  reacts  with  silver  nitrate.  These 
two  acids  may  be  recognised  by  the  following 
tests  : A portion  of  the  nitric  acid  is  diluted 
with  about  three  times  its  bulk  of  distilled  water. 
To  a portion  of  this  is  added  a solution  of  barium 
chloride.  If  sulphuric  acid  is  present  a white 
precipitate  is  produced  of  barium  sulphate,  in- 
soluble in  hydrochloric  acid.  To  another  portion 
of  the  diluted  commercial  sample,  a solution  of 
silver  nitrate  is  added.  If  a white  precipitate 
is  obtained — silver  chloride,  soluble  in  ammonia 
— this  shows  the  presence  of  hydrochloric  acid. 

Tests  for  Nitric  Acid  and  Nitrates. — Nitrates 
and  nitric  acid  may  be  recognised  as  follows  : — 
(1)  A nitrate  treated  with  a drop  or  two  of  con- 
centrated sulphuric  acid  gives  a pungent  odour 
of  nitric  acid.  On  dropping  in  a few  pieces  of 
metallic  copper,  brown  fumes  of  nitrogen  per- 
oxide are  evolved. 

(2)  A solution  of  the  nitrate  is  mixed  with  a 
solution  of  ferrous  sulphate  in  the  cold,  in  a test 
tube.  A drop  of  concentrated  sulphuric  acid  is 
now  very  cautiouly  added  down  the  side  of  the 
tube.  The  acid  sinks  to  the  bottom.,  and  where  it 
meets  the  solution  of  the  nitrate  and  sulphate 
a brown  ring  is  produced.  This  forms  a very 
delicate  test  for  a nitrate,  and  is  usually  spoken 
of  as  the  “ brown  ring  test.’^ 

Ammonia. — Ammonia  is  a gaseous  compound, 
containing  three  atoms  of  hydrogen  and  one  atom 
of  nitrogen  in  the  molecule,  consequently  its 
formula  is  NH3.  It  may  be  prepared  by  heating 
any  compound  containing  ammonia,  such  as  am- 
monium sulphate,  oxalate,  chloride,  etc.,  with 


NITROGEN  COMPOUNDS  EMPLOYED. 


95 


quicklime  or  caustic  potash.  It  is  conveniently 
prepared  in  the  following  manner  : A hard-glass 
flask  is  taken,  fitted  with  a cork  and  delivery  tube, 
and  arranged  as  in  Fig.  29. 

A mixture  of  any  ammonium  salt  and  pow- 
dered quicklime  is  then  introduced  into  the  flask 
and  heated.  Reaction. — 


Fig.  29. — Preparation  of  Aramonia, 


CaO  -f  2NH,C1  = CaCF  -f  H^O  -f  2NH3 

Lime.  + Ammonium  = Calcium  Water,  -f  Ammonia 

chloride.  cliloride.  gas. 

Notice  that  the  gas  is  invisible  and  has  a pungent 
odour.  To  a jar  of  the  gas  add  a little  red  lit- 
mus solution.  Observe  that  it  is  instantly  turned 
blue,  showing  that  the  ammonia  is  an  alkali. 

Place  a jar  of  ammonia  gas,  mouth  downwards, 
in  a basin  of  water.  The  water  instantly  rushes 
up  the  jar  and  completely  fills  it.  This  solution 
of  ammonia  gas  in  water  is  termed  ammonium 
hydroxide,  and  it  is  in  this  form  that  ammonia 
is  used. 


96 


PHOTOGEAPHIC  CHEMISTEY. 


NH3  + H3O  = NH,OH 

Ammonia  gas.  Liquid  ammonia. 

Photographic  Uses  of  Ammonia. — Ammonia 
solution  is  one  of  the  alkalis  used  in  development 
with  pyrogallic  acid.  Owing  to  the  readiness 
with  which  ammonia  combines  with  silver  nitrate, 
it  is  used  in  the  preparation  of  gelatino-bromide 
emulsion  by  the  ammonia  process.  Ammonia 
also  has  the  property  of  dissolving  silver  chloride 
and  bromide,  but  not  the  iodide. 

AgCl  AgBr  Agl 

Very  soluble  j.  Not  so  Insoluble  in 

in  ammonia.  ^ soluble.  ^ ammonia. 

Ammonium  Nitrate  (NH4NO3). — If  this  com- 
pound is  added  to  water,  a considerable  lowering 
of  temperature  is  obtained,  as  the  salt  dissolves. 
This  has  been  suggested  for  preventing  frilling 
during  development  in  hot  countries.  The  de- 
veloping dish  is  placed  in  a somewhat  larger  dish 
containing  water,  and  to  this  is  added,  from  time 
to  time,  solid  ammonium  nitrate.  The  ammonium 
nitrate  can  be  recovered  by  simply  evaporating 
the  solution  to  dryness  on  a water  bath. 

Acids,  Alkalis  or  Bases,  and  Salts. — It  will 
be  convenient  here  to  have  a few  definitions  of 
what  is  understood  by  the  terms  “ acids,^^  “ bases 
or  alkalis,”  and  “ salts.” 

Acids. — These  are  bodies  which  possess  in  a 
more  or  less  marked  degree  the  following  proper- 
ties : — 

(1)  They  have  a sharp,  sour  taste. 

(2)  Blue  vegetable  colouring  matters,  such  as 
litmus,  are  turned  red. 

(3)  With  sodium  carbonate  they  cause  effer- 
vescence due  to  liberation  of  CO2. 

(4)  They  neutralise  alkalis  forming  salts. 

Acids  in  all  cases  contain  hydrogen,  and  they 

may  be  grouped  according  to  the  number  of  atoms 
of  this  element  present  in  the  molecule.  Acids 


NITEOGEN  COMPOUNDS  EMPLOYED. 


97 


containing  one  replaceable  hydrogen  atom  are 
termed  monobasic  acids.  For  example,  HCl, 
HBr,  HI,  HNO3.  Those  containing  two  replace- 
able hydrogen  atoms  are  termed  dibasic  acids,  as 
H2SO4,  H2C0O4  (oxalic  acid),  H2SO3  (sulphurous 
acid),  H2CO3  (carbonic  acid).  Acids  containing 
three  or  four  replaceable  hydrogen  atoms  are 
termed  tri-,  tetra-,  basic  acids,  etc. 

Acid  Anhydrides. — If  a molecule  of  water  is 
abstracted  from  an  acid,  a substance  is  obtained 
which  is  termed  the  anhydride  of  that  acid.  For 
example  : — 

HNO3  _ JJ  Q ^ Q 

HNO3  ^ ^ 

2 molecules  of  nitric  acid.  Nitric  anhydride. 

H2SO4  - H2O  = SO3 

Sulphuric  acid.  Sulphuric  anhydride. 

H2SO3  - H2O  = SO2 

Sulphurous  acid.  = Sulphurous  anhydride. 

Of  course,  acids  like  hydrochloric  acid,  having 
the  formula  HCl,  do  not  form  anhydrides.  An- 
hydride formation  is  only  possible  when  the  acid 
contains  oxygen. 

Alkalis  or  Bases. — These  substances  have  pro- 
perties opposed  to  those  of  an  acid  : — 

(1)  They  have,  as  a rule,  a “ soapy  ” taste. 

(2)  Reddened  vegetable  colouring  matters  are 
turned  blue. 

(3)  They  absorb  carbon  dioxide. 

(4)  They  neutralise  acids  partially  or  entirely, 
forming  salts. 

These  compounds  are  usually  divided  into 
three  classes  : — 

{a)  Metallic  oxides,  such  as  CaO,  Na20,  K2O. 

{h)  Metallic  hydroxides  or  hydrates.  These 
are  compounds  containing  a metal  united  with 
one  or  more  hydroxyl  groups.  Hydroxyl  is  the 
name  applied  to  a monovalent  grou]i  containing 
one  atom  of  hydrogen  and  one  of  oxygen  (OH), 
a 


98 


PHOTOGRA.PHTC  CHEMISTRY. 


For  example,  sodium  hydrate,  NaOH;  barium 
hydrate,  Ba(OH)2;  ferric  hydrate,  Fe(OH)g. 

(c)  Certain  compounds  containing  hydrogen, 
the  chief  of  which  is  NHg,  ammonia. 

Salts. — These  are  substances  which  result  by 
treating  an  alkali  or  base  with  an  acid. 

They  are  obtained  by  replacing  one  or  more 
atoms  of  hydrogen  in  the  acid  by  a metal  or  some 
group  of  atoms  playing  the  part  of  a metal,  such 
as  the  radical  ammonium'  (NH^). 

Salts  of  monobasic  acids  will  be  of  one  kind 
only,  because  they  contain  only  one  replaceable 
hydrogen  atom.  For  example  : — 


Acid. 

Silver  Salt. 

Sodium  Salt. 

Ammonium 

Salt. 

HNO3,  Nitric  acid 
HCl,  Hydrochloric  ) 
acid  ) 

AgN03 

AgCl 

NaNOg 

NaCl 

NH4NO3 

NH4CI 

HBr,Hydrobromic  \ 

acid  1 

HI,  Hydriodicacid 

AgBr 

NaBr 

NH^Br 

Agl 

Nal 

NH4T 

Dibasic  acids,  containing,  as  they  do,  two  replace- 
able hydrogen  atoms,  form  two  classes  of  salts 


Sulplmric  acid. 


>SO, 

>so. 

h/ 

Na/ 

Sodium  hydrogen 

Di-sodium 

sulpliate. 

sulphate. 

Acid  Salts. — Salts  obtained  by  replacing  only 
part  of  the  hydrogen  of  the  acid  are  termed  acid 
salts,  because  they  still  retain  some  of  their  acid 
character.  They  are  also  designated  by  the  pre- 
fix “ hi  ” ; thus  the  first  compound  sodium  hydro- 
gen sulphate  is  also  known  as  sodium  bisulphate. 
Compounds  obtained  by  replacing  all  the  hydro- 
gen atoms  in  the  acid  are  termed  normal  or 
neutral  salts,  because,  speaking  generally,  they 


NITROGEN  COMPOUNDS  EMPLOYED. 


99 


' are  neither  acid  nor  alkaline  in  their  behaviour. 
The  potassium  salts  of  oxalic  acid  are  : — 

Oxalic  acid.  Potassium  Normal  or  neutral 

binoxalate.  potassium  oxalate. 

Tribasic  acids,  containing  three  replaceable 
hydrogen  atoms,  form  three  classes  of  salts. 
These  are  termed  primary,  secondary,  and  ter- 
tiary salts.  For  instance,  citric  acid  is  a tri- 
basic acid,  and  forms  three  ammonium  salts  : — 

H\  nh^\ 

H—yCeOyHs  II  NH^-^CjjOyHs  NH^-^CeO^Hr 

H / II  / H / 

Citric  acid.  Primary  Secondary 

ammonium  ammonium 

citrate.  citrate. 

V , ^ 

Acid  salts. 

Preparation  of  Salts. — It  will  be  convenient 
h(3re  to  consider  a few  general  equations  repre- 
senting the  formation  of  salts  : — 

(1)  Action  of  acid  on  metal. 

Zn  -P  H0SO4  = ZnSO^  -1-  H2 

Zinc.  Dilutesulphuric  _ Zinc  sulphate.  Hydrogen, 

acid. 

(2)  Action  of  acid  on  oxide  of  metal. 

CuO  -f  H2SO4  = CuSO,  -t-  H.O 

Copper  Sulphuric  _ Copper  Water, 

oxide.  acid.  sulphate. 

(3)  Action  of  acid  on  carbonate  of  metal. 

Na,CO,  + H,SO.  = Na,SO.  + H,0  + CO, 

Sodium  , Sulphuric  _ SodiuTn  , Water.  Carbon 

carbonate.  acid.  sulphate.  dioxide 

(4)  Action  of  acid  on  the  hydroxide. 

H,C,0.  + 2NH,OH  = (NH.),CA  + 2H,0 

Oxalic  , Ammonium  _ Ammonium  Water, 

acid.  hydroxide.  oxalate. 

(5)  Many  salts  are  obtained  by  allowing  one 
salt  to  act  upon  another,  whereby  the  acid  part  of 


N1I4/ 

Tertiary 

ammoniun 

citrate. 


Neutral 
S salt. 


100 


PHOTOGRAPHfO  CHEMISTRY. 


each  compound  is  mutually  exchanged.  This  is 
termed  double  decomposition. 

AgNOa  + NaBr  = AgBr  +NaNOa 

Silver  Sodium  _ Silver  , Sodium 

nitrate.  bromitle.  bromide.  ^ nitrate. 

CdBr^  + 2AgN03  = CclCNOj)^  + 2AgBr 

Cadmimu  , Silver  _ Ca<lmiuiu  , Silver 

bromide.  nitrate.  ~ nitrate.  bromide. 

Carbonic  Acid  and  the  Carbonates. — When 
carbon,  or  substances  containing  carbon,  undergo 
oxidation,  oxide  of  carbon  is  obtained. 

The  common  name  for  this  compound  is  car- 
bonic acid.  Strictly  speaking,  it  is  the  anhy- 
dride of  carbonic  acid.  Photographers  working 
in  small,  badly-ventilated  dark-rooms  are  often 
conscious  of  a drowsy  feeling  stealing  over  them, 
which  in  most  cases  develops  into  a bad  attack  of 
headache.  This  is  due  to  the  presence  of  the  car- 
bon dioxide.  The  tissues  of  the  body  contain 
carbon,  and  during  the  operation  of  breathing 
this  undergoes  oxidation.  That  this  is  so  is 
readily  seen  by  breathing  through  a glass  tube 
l^assing  into  a clear  solution  of  limewater.  In 
a very  short  time  the  solution  becomes  turbid 
owing  to  the  formation  of  chalk. 

Ca(OH),  + CO,  = CaC03  4-  H,0. 

Limewater.  = Chalk. 

Preparation  of  Carbonic  Acid. — The  gas  is 
readily  obtained  by  treating  any  carbonate  with 
any  acid.  The  apparatus  required  is  the  same  as 
that  used  for  the  preparation  of  ^hydrogen  (see 
Fig.  24,  p.  53).  Some  sodium  carbonate  is  intro- 
duced into  the  flask  and  then  dilute  hydrochloric 
acid  is  poured  down  the  thistle  funnel.  On  intro- 
ducing a light  into  the  gas  it  is  found  to  be  ex- 
tinguished. If  a piece  of  magnesium  ribbon  is 
ignited  and  lowered  into  a jar  of  the  gas,  it 
burns  with  its  usual  brilliant  light,  accompanied 
by  much  spluttering.  If  the  jar  is  then  examined 


NITROGEN  COMPOUNDS  EMPLOYED. 


101 


a white  powder  is  observed,  mixed  with  small 
black  specks.  The  white  .compound  is  oxide  of 
magnesium,  and  the  black  is  carbon.  Equa- 
tion : — 

CO2  + 2Mg  = 2MgO  + C 

Magnesium  + Carbon, 
oxide. 

Pass  some  of  the  gas  into  water,  and  then  test 
with  a piece  of  blue  litmus  paper  (made  by  soak- 
ing blotting-paper  in  blue  litmus  solution  and 
allowing  to  dry).  It  turns  red,  showing  the  pres- 
ence of  an  acid. 

CO,  + H,0  = H2CO3 

Carbonic  anbydride.  Carbonic  acid. 

The  free  acid,  H0CO3,  is  too  unstable  to  exist  ex- 
cept in  solution,  and  all  attempts  to  isolate  it 
simply  result  in  the  formation  of  its  anhydride 
CO3. 

Garbonates. — The  salts  of  this  acid  are  known 
as  the  carbonates,  and  are  of  frequent  use  in 
photographic  work,  owing  to  their  alkaline  be- 
haviour. Carbonic  acid  is  seen  to  be  dibasic, 
consequently  it  will  form  two  series  of  salts  : 


Sodium  Normal  sodium 

bicarbonate.  carbonate. 


Both  these  compounds  are  alkaline. 

If  a solution  of  the  bicarbonate  is  heated  for 
some  time  it  loses  carbon  dioxide  and  passes  into 
the  normal  carbonate. 

CO,  <■  CO3  = Na.CO,  + H,0  + CO.. 

“ \ 1 H H/  - t 3 

Ammonium  CarJ)onafe  (NH^)oCO,. — This  com- 
pound is  used  in  certain  photographic  operations, 
and  in  some  cases  it  acts  rather  abnormally. 
This  is  due,  in  most  cases,  to  the  fact  that 
ordinary  commercial  ammonium  carbonate  or 


102 


PHOTOGRAPHIC  CHEMISTRY. 


sesqui-carbonate  of  ammonia  is  generally  a mix- 
ture  of  normal  and  bicarbonates  of  ammonium, 
together  with  another  body  known  as  ammonium 
carbamate.  The  formulae  of  these  compounds 
are  : — 

^h‘>  CO3 

Aramoniuin 
bicarbonate, 

AH  these  compounds  are  obtained  when  carbon 
dioxide  acts  upon  ammonia  gas. 

Conversion  of  Bicarbonate  and  Carbamate  of 
Ammonia  to  Normal  Carbonate. — If  a little  liquid 
ammonia  is  added  to  the  bicarbonate  and  car- 
hamate  they  pass  ir  *;0  the  normal  carbonate  aa 
follows  : — 


™.>C03 

NH3 

NHy 

+ 

H,0 

1! 

0 

0 

M 

Ammonium  carbonate 

should 

be  preserved  in 

well-stoppered  bottles,  otherwise  it  loses  am- 
monia and  passes  into  the  bicarbonate. 


NH4 

NH, 


> CO, 


Normal 

ammonium 

carbonate. 


NH, 

NH.. 

Ammonium 

carbamate. 


> CO, 


103 


CHAPTER  IX. 

THE  HALOGENS  AND  HALOID  SALTS. 

Explanation  of  Term,  “ HalogensE — The  four 
elements  chlorine,  bromine,  iodine,  and  fluorine 
are  very  much  alike  in  their  chemical  behaviour, 
and  as  a group  they  are  spoken  of  as  the  halo- 
gens or  haloid  elements.  The  word  halogen 
means  literally  “ salt  former,”  and  was  applied 
to  the  four  elements  mentioned  because  they  form 
salts  having  similar  properties.  They  are  mono- 
valent, and  their  compounds  with  hydrogen  and 
the  metals  are  named  in  the  following  manner  : — 

HCl  Hydrochloric  acid  or  hydrogen  chloride. 

HBr  Hydrobromic  acid  or  hydrogen  bromide. 

HI  Hydriodic  acid  or  hydrogen  iodide. 

HE  Hydrofluoric  acid  or  hydrogen  fluoride. 

Metal  + chlorine  is  termed  a chloride. 

„ + bromine  „ „ bromide. 

„ + iodine  „ „ iodide. 

„ + fluorine  „ „ fluoride. 

Chlorine. — As  this  element  plays  a very  im- 
portant part  in  photography  its  preparation  and 
properties  are  given  below.  This  gas  is  best  pre- 
pared in  the  open  air,  and  great  care  must  be 
taken  to  avoid  breathing  it.  A small,  wide- 
mouthed flask,  of  about  4-oz.  capacity,  is  taken, 
having  a tightly-fitting  cork  pierced  with  two 
holes.  Through  one  hole  passes  a thistle  funnel, 
and  through  the  other  a delivery  tube  bent  as  in 
Fig.  30.  About  half  fill  the  flask  with  concen- 
trated hydrochloric  acid,  and  then  add  about  half 
this  quantity  of  manganese  dioxide.  Cautiously 
heat  the  flask  and  collect  the  gas.  As  it  has  a slight 


104 


PHOTOGRAPHIC  CHEMISTRY. 


greenish-yellow  colour  it  is  easy  to  see  when  a 
jar  is  full. 

Equation  : 

MnO,  4HC1  = MnCL  + 2H,0  -f-  Cl^ 

Manganese  Manganese 

dioxide.  cldoride. 

JE xijeriments  with  Chlorine. — Experiment  1 : 
Introduce  a lighted  taper  into  a jar  of  the  gas. 
Notice  that  the  taper  burns,  but  with  a very 
smoky  flame. 

Experiment  2 ; Pour  a few  drops  of  turpen- 
tine on  to  a piece  of  tissue  paper  and  drop  this 


Fig.  30. — Preparation  of  Chlorine. 


into  another  jar  of  the  gas.  If  a fair  amount  of 
chlorine  is  present,  the  paper  instantly  bursts 
into  flame.  The  turpentine  and  the  wax  of  the 
taper  contain  the  elements  of  hydrogen  and  car- 
bon. The  chlorine  abstracts  this  hydrogen,  set- 
ting free  the  carbon,  and  this  takes  place  so 
rapidly  in  the  case  of  the  turpentine  that  it  is 
ignited. 

Experiment  3 : Damp  a piece  of  coloured  cloth 
or  paper,  and  allow  it  to  remain  for  some  time 
in  a jar  of  chlorine.  The  colour  will  be  found 
to  be  destroyed.  Hence  chlorine  is  a bleaching 
agent.  All  the  organic  colouring  matters  used 
in  photography  are  bleached  by  chlorine.  The 
bleaching  action  of  chlorine  is  largely  influenced 


THE  HALOGENS  AND  HALOID  SALTS.  105 


by  the  presence  of  moisture.  Dry  chlorine  has 
practicably  no  bleaching  properties. 

Experiment  4 : Make  a little  starch  paste  and 
add  to  it  a solution  of  potassium  iodide.  Dip 
a piece  of  blotting  paper  into  this  solution  and 
introduce  into  a jar  of  the  gas.  Observe  that  it 
is  instantly  turned  blue.  This  forms  a delicate 
test  for  the  presence  of  chlorine.  The  chlorine 
acts  upon  the  potassium  iodide  solution,  forming 
potassium  chloride  and  free  iodine.  This  iodine 
then  forms  a blue-coloured  compound  with  the 
starch  : 

2KI  -f  Cl,  = 2KC1  -f  I,. 

Chlorine  also  possesses  the  property  of  displacing 
bromine  from  its  solutions.  This  is  readily  seen 
by  passing  some  of  the  gas  into  a dilute  solution 
of  potassium  or  ammonium  bromide.  A brownish- 
red  colour  is  instantly  produced. 

2KI  -f  CL  = 2KC1  + Br„ 

2NH,Br  + CL  = 2NH,C1  -h  Br,. 

Influence  of  TAght  on  Chlorine  in  Solution. — 
The  chemical  action  of  chlorine  is  materially  in- 
creased by  the  action  of  sunlight.  For  instance, 
a mixture  of  equal  volumes  of  chlorine  and  hydro- 
gen combine  in  direct  sunlight  with  a violent 
explosion  to  produce  hydrochloric  acid. 

H,  -h  CL  = 2HC1 

In  diffused  daylight  the  action  is  a gradual  one, 
unaccompanied  by  an  exposion.  In  the  dark,, 
especially  if  the  gases  are  dry,  no  action  takes 
place  at  all.  This  mixture  of  chlorine  and  hydro- 
gen constituted  one  of  the  first  actinometers,  as 
the  amount  of  hydrochloric  acid  produced  is  pro- 
portional to  the  light  intensity.  It  has  already 
been  stated  that  chlorine  has  a great  affinity  for 
the  hydrogen  in  compounds  containing  that  ele- 
ment. Thus  water  is  decomposed  into  hydro- 
chloric acid  and  oxygen,  and  the  greatest  decom- 
position is  effected  in  the  presence  of  direct 


106 


PHOTOGKA.PHIC  CHEMISTRY. 


sunlight.  If  a glass  jar  containing,  and  standing 
over  chlorine  water,  be  exposed  to  the  sunlight, 
a gas  is  evolved  which  collects  at  the  top  of  the 
jar,  and  on  examination  it  is  found  to  be  oxygen. 
In  diffused  daylight  the  action  is  slower,  and  in 
the  dark  it  almost  ceases. 

The  main  course  of  the  reaction  may  be  repre- 
sented as  follows  : — 

2H2O  + 2CI2  = 4HC1  + O2 
Small  quantities,  however,  of  compounds  contain- 


Fig’.  31. — Experiment  showing  Action  of 
Light  on  Silver  Chloride. 

ing  chlorine,  hydrogen,  and  oxygen  are  produced 
at  the  same  time. 

Action  of  Light  on  Metallic  Chlorides. — Many 
metallic  chlorides  lose  chlorine  on  exposure  to 
bright  light,  especially  in  the  presence  of  mois- 
ture. For  instance,  if  a quantity  of  moist  silver 
chloride  is  exposed  to  bright  sunlight  for  about 
a week,  in  a glass  tube,  as  shown  in  Fig.  31, 
chlorine  is  found  to  be  present  in  the  water  con- 
tained in  the  beaker.  Mercuric  chloride  passes 
to  mercurous  chloride  if  its  solution  is  exposed  to 
bright  sunlight  for  a short  period. 

4HgCl2  + 2H2O  = 2HgCl  + 4HC1  + O, 


THE  HALOGENS  AND  HALOID  SALTS.  107 


The  chlorine,  instead  of  being  liberated,  attacks 
the  water,  as  described  above,  and  liberates  the 
oxygen. 

Bromine  is  a heavy,  reddish-brown 
liquid,  with  an  exceedingly  penetrating  odour 
(hence  the  name  bromine,  from  bromos  = a 
stench).  Bromine  can  be  obtained  by  passing 
chlorine  into  a solution  of  a bromide.  It  is  ob- 
tained commercially  by  passing  chlorine  into 
magnesium  bromide. 

MgBra  + CI2  = MgCla  + Bl'2 

Magnesium 

bromide. 

In  its  properties  bromine  is  a perfect  analogue 
of  chlorine,  though  weaker  than  that  element  in 
chemical  activity.  In  contact  with  organic 
matter  containing  hydrogen,  like  chlorine,  it 
abstracts  it,  forming,  however,  hydrobromic  acid. 
It  is  interesting  to  notice  that  bromine  does  not 
combine  with  hydrogen  in  the  presence  of  sun- 
'i!ight.  With  starch  paste,  bromine  produces  a 
yellow  colour. 

Iodine. — This  element  is  a blackish  solid.  Of 
late  years  large  quantities  have  been  obtained 
from  Chili  saltpetre,  as  this  substance  contains 
sodium  iodate.  Iodine  has  a characteristic 
odour;  it  stains  the  skin  and  organic  matter 
generally  a brown  colour.  It  may  be  displaced 
from  its  solutions  by  passing  chlorine  through 
them.  2KI  + Cl,  = 2KC1  + I, 

The  deep  blue  colour  imparted  to  starch  solution 
is  characteristic  of  iodine.  It  does  not  combine 
with  hydrogen  in  the  sunlight,  and  has  not  the 
property,  like  chlorine  and  bromine,  of  removing 
hydrogen  from  its  compounds.  Iodine  may  be 
purified  by  submitting  it  to  sublimation. 

Fluor ine.~T\i\^  element  is  not  so  important 
as  the  other  haloids,  as  only  one  or  two  of  its 
compounds  are  used  in  photographic  work.  It 


108 


PHOTOGRAPHIC  CHEMISTRY. 


will  be  sufficient  here  to  mention  that  it  is  the 
most  active  of  all  elements,  and  very  difficult  to 
isolate  in  the  free  condition. 

Hydrogen  Compounds  of  the  Halogens — 
Hydrogen  Chloride. — This  compound,  as  already 
mentioned,  is  formed  by  the  direct  union  of  hj^dro- 
gen  and  chlorine  in  the  presence  of  sunlight.  It 
is  interesting  to  notice  that  this  union  is  also 
brought  about  by  the  light  given  out  by  burning 
magnesium.  This  acid  is  readily  prepared  by 
treating  any  chloride  with  strong  sulphuric  acid. 


Na  I Cl  + H 
H 

Salt. 


/ so. 

Sulphuric 

acid. 


Sodium  + 
bisulpliate. 


HCl 

Hydro- 
chloric acid. 


This  reaction  may  be  carried  out  with  the  same 
apparatus  as  described  under  “ Chlorine.’’  It  is 
best  to  work  out  of  doors,  taking  care  not  to 
breathe  the  gas. 

Experiments  with  Hydrogen  Chloride. — Ex- 
periment 1 : Place  a jar  of  the  gas,  mouth  down- 
w'ards,  in  a vessel  containing  water.  Observe  that 
the  water  very  quickly  dissolves  the  gas,  and  rises 
to  the  top  of  the  jar. 

Experiment  2 : Make  a solution  of  the  gas  in 
water,  and  treat  successive  portions  with  : — 

{a)  Blue  litmus  paper  or  solution. 

(6)  Sodium  carbonate  solution. 

(c)  Silver  nitrate  solution. 

Observe  that  {a)  turns  red  and  {h)  gives  off 
carbon  dioxide.  These  experiments  show  the  acid 
character  of  hydrogen  chloride.  In  {c)  a white 
flocculent  precipitate  of  silver  chloride  is  ob- 
tained. 


H 


Cl  + Ag  I NO3  = AgCl  + HNO3 


In  photographic  work,  hydrochloric  acid — that 
is,  a solution  of  the  gas  in  w^ater — is  used  for 
dissolving  out  ferrous  oxalate  from  platinotype 
prints,  for  the  preparation  of  certain  chlorides, 


THE  HALOGENS  AND  HALOID  SALTS.  109 


and  for  a large  number  of  minor  uses.  The  com- 
mercial acid  is  really  a strong  solution  of  the 
gas  in  water,  and  the  strong  acid  contains  about 
33  per  cent,  of  HCl.  This  acid  is  also  known 
under  the  name  of  muriatic  acid.  The  muriates 
are  the  chlorides.  As  ordinarily  obtained  the 
commercial  acid  contains  iron  and  small  quan- 
tities of  sulphuric  acid. 

Tests  for  Impurities  in  Hydrochloric  Acid. — 
Impurities  may  be  detected  as  follows  : — 

Iron. — A strong  yellow  colour  denotes  the  pres- 
ence of  iron.  A portion  of  the  acid  is  diluted, 
and  to  this  is  added  a solution  of  potassium 
ferrocyanide  (yellow  prussiate).  The  presence  of 
iron  is  shown  by  the  formation  of  a blue  precipi- 
tate of  Prussian  blue. 

Sulphuric  Acid. — A portion  of  the  commercial 
acid  is  diluted  with  distilled  water,  and  then  a 
solution  of  barium  chloride  added.  White  pre- 
cipitate, insoluble  on  warming,  shows  the  pres- 
ence of  sulphuric  acid.  If  the  acid  is  submitted 
to  distillation  these  impurities  are  removed. 

Aqua  lieyia. — A mixture  of  one  volume  of 
nitric  acid  and  three  volumes  of  concentrated 
hydrochloric  acid  is  known  as  aqua  regia,  as  such 
a mixture  has  the  power  of  dissolving  gold  and 
platinum,  which  neither  of  the  acids  alone  is 
capable  of  doing.  The  powerful  solvent  action  of 
the  aqua  regia  is  probably  due  to  the  presence  of 
free  chlorine,  and  a body  known  as  nitrosyl 
chloride  NOCl. 

3HC1  + HNO3  = 2H3O  + NOCl  + CI3 
A mixture  of  sodium  nitrite  solution  and  hydro- 
chloric acid  has  also  the  property  of  dissolving 
gold.  The  equation  representing  the  change  is 
probably  as  follows  : — 

3NaN02  + 6HC1  + O.  = 3NaCl  4-  NOCl  + 3H3O 
-F  N3O,  + CL 

Chlorides  Used  in  Fhotoyraphy. — Sodium 


110 


PHOTOGRAPHIC  CHEMISTRY. 


Chloride^  or  common  salt  (NaCl).  This  was  one 
of  the  earliest  of  the  fixing  agents,  as  it  has  the 
power  of  dissolving  silver  chloride,  but  cannot 
be  compared  to  “ hypo.”  It  was  at  one  time 
largely  used  for  “ salting  ” albumenised  paper, 
to  bring  about  the  formation  of  silver  chloride. 

NaCl  + AgN03  = AgCl  + NaN03 
It  has  also  many  minor  uses  in  photography. 

Ammonium  Chloride  (NH^Cl). — This  com- 
pound is  used  for  “ salting  ” albumenised  paper, 
and  the  reaction  taking  place  is  : 

NH,C1  + AgN03  = AgCl  + NH,N03 
It  may  also  be  used  for  preparing  strong  solu- 
tions of  mercuric  chloride.  This  mercuric  chloride 
is  not  very  soluble  in  water,  but  it  is  much  more 
soluble  in  solutions  of  ammonium  chloride.  The 
strongest  solution  is  obtained  when  the  salts  are 
one  molecule  of  mercuric  chloride  to  six  molecules 
of  ammonium  chloride. 

Ferric  Chloride  (FeClg  or  FesClg). — This  com- 
pound is  obtained  by  dissolving  iron  in  hydro- 
chloric acid,  and  then  passing  chlorine  gas 
through  the  ferrous  chloride  so  formed. 

Fe  + 2HC1  = FeCl^ 

Feri'ous  chloride. 

2FeCl,  + CI3  =2FeCl3 

Ferric  cliloride. 

If  the  ferrous  chloride  is  exposed  to  the  air  , or 
has  air  blown  through  it,  it  slowly  oxidises  to 
ferric  chloride.  Ferric  chloride  nearly  always 
contains  free  hydrochloric  acid,  but  the  presence 
of  the  acid  does  not  render  it  harmful;  in  fact, 
in  certain  photo-mechanical  processes  it  is  an 
advantage.  Of  course,  it  must  not  contain  too 
much  acid.  If  too  acid,  this  may  be  neutralised 
in  the  following  manner  : A quantity  of  the  ferric 
chloride  is  taken  and  treated  with  ammonia.  This 
produces  a gelatinous  precipitate  of  ferric 
hydrate. 


THE  HALOGENS  AND  HALOID  SALTS.  Ill 


Cl 

NH,. 

Cl  4-  NH. 

Cl 

NH, 

OH  / OH 

OH  = Fe/OH  + SNH^CI. 

OH  \ OH 


This  precipitate  is  well  washed  by  decantation, 
and  then  carefully  added  to  the  ferric  chloride  to 
be  neutralised  till  the  right  degree  of  neutralisa- 


tion has  been  reached. 


Cl 

Cl  = FeCls  + 3H2O. 

Cl 

Acid  present  in 
the  ferric  chloride. 

Gold  Chloride  (AuClg,  Auric  chloride). — This 
is  obtained  by  dissolving  gold  in  aqua  regia,  or 
by  treating  the  metal  with  chlorine. 

(1)  Au  + HNO3  3HC1  = AUCI3  4-  2H3O  -F 
NO,  etc. 

(2)  2Au  4-  3CL  = 2AUCI3. 

On  evaporating  its  solutions  in  the  presence  of 
hydrochloric  acid  it  crystallises  in  yellow-coloured 
needles  containing  one  molecule  of  hydrochloric 
acid  and  four  molecules  of  water  of  crystallisa- 
tion. It  is  in  this  form  that  it  is  bought. 

HAuCl^  4H0O.  Chlorauric  acid. 


Fe^ 


OH 

H 

OH  4-  H 

OH 

H 

As  it  is  important  that  the  gold  toning  bath 
should  bo  neutral,  this  hydrochloric  acid  is 
removed  by  adding  a small  quantity  of  chalk  to 
the  gold  solution.  By  evaporating  a solution  of 
gold  and  sodium  chlorides  to  the  crystallising 
point,  a double  neutral  chloride  of  sodium  and 
gold  separates,  which  also  has  the  property  of 
not  being  deliquescent — that  is,  of  absorbing 
water.  ^-^01  + AuCf  = NaAuCl, 

Scdiutn  cliloraurate. 

Platinic  Chloride  (PtCl^). — This  compound  is 
obtained  by  dissolving  platinum  in  aqua  regia ; it 
is  essentially  the  nascent  chlorine  which  attacks 
the  metal. 


112  PHOTOGRAPHIC  CHEMISTRY. 

Pt  + 2CL  = PtCl, 

When  heated  carefully  to  200°  C.  it  breaks  down 
into  platinous  chloride,  PtCl2.  Platinous  chloride 
forms  a double  chloride  with  potassium,  and  this 
compound  is  used  in  platinotype  printing. 

2KC1  + PtCL  = K^PtCl,. 

Calcium  Chloride  (CaCla). — This  compound  is 
used  as  a drying  agent  to  prevent  damp  from 
reaching  platinum  paper.  It  is  obtained  by 
treating  chalk  with  hydrochloric  acid,  evaporat- 
ing to  dryness,  and  then  fusing  the  resulting  com- 
pound. 

CaC03  + 2HC1  = CaCl^  + H,0  + CO^ 

Chalk. 

Tlydrohromic  Acid  (HBr)  and  Hydriodic  Acid 
(HI). — These  compounds  are  both  gases,  and  fume 
strongly  in  the  air.  Both  are  soluble  in  water, 
and  it  is  in  this  form  that  they  are  met  with  in 
practice.  They  are  not  obtained  by  treating  a 
bromide  or  an  iodide  with  strong  sulphuric  acid. 
If  sulphuric  acid  is  added  to  an  iodide,  hydriodic 
acid  is  first  formed,  but  it  instantly  attacks  the 
sulphuric  acid  and  breaks  it  down  to  sulphur 
dioxide. 

(1)  KI  + g>  SO,  = g>SO,  + HI 

(2)  H,SO,  + 2HI  = 2H,0  + SO,  + I, 

Hydrobromic  acid  behaves  in  a similar  manner; 
hence  it  is  important  to  remember  that  both  these 
acids  are  powerful  reducing  agents,  owing  to  the 
readiness  with  which  they  give  up  their  hydrogen. 

Bromides  and  Iodides  Used  in  Photography. — 
These  compounds  may  be  obtained  by  adding  the 
halogen  acid,  HBr  or  HI,  to  the  hydrate  or  car- 
bonate of  the  metal. 


THE  HALOGENS  AND  HALOID  SALTS.  113 


Sodium  bromide  and  iodide 
Potassium  bromide  and  iodide  .. 
Ammonium  bromide  and  iodide.. 
Cadmium  bromide  and  iodide  .. 
Zinc  bromide  and  iodide 


NaBr  and  Nal. 
KBr  and  KL 
NH^Br  and  NH  J. 
CdBr^  and  Cdl,. 
ZnBrg  and  Znla. 


NaOH  + HI  = Nal  + H,0 
K2CO3  + 2HBr  = 2KBr  + CO^  + H.O 

They  are  largely  used  in  photography  for  two 
main  purposes— (a)  collodion  and  gelatine  emul- 
sion making,  and  (b)  as  restrainers  in  develop- 
ment. In  the  emulsions  the  silver  nitrate  present 
reacts  with  a mixture  of  various  bromides  and 
iodides  to  form  the  corresponding  bromides  and 
iodides  of  silver. 

2AgN03  + CdBr,  = 2AgI  + Cd(N03)2 
2AgN03  + Znl^  = 2AgI  + Zn(N03)3 
They  are  used  as  restrainers  in  development  be- 
cause they  prevent  the  too  rapid  reduction  of  the 
silver  haloid  on  the  plate.  The  silver  and  mer- 
cury haloids  are  dealt  with  in  Chapter  XI. 

Hydrofluoric  Acid  (HF). — This  compound  is 
a gas  having  a very  sharp,  disagreeable  odour. 
It  has  a most  powerful  solvent  action  on  glass, 
and  consequently  it  cannot  be  prepared  or  kept 
in  glass  vessels.  In  practice  it  is  made  by  decom- 
posing calcium  fluoride  with  strong  sulphuric 
acid  in  leaden  retorts,  and  passing  the  gas  into 
water.  The  aqueous  solution  is  then  put  oa  the 
market  in  rubber  bottles. 

Preparation  of  Hydrofluoric  Acid. — Procure  a 
small  quantity  of  calcium  fluoride  and  introduce 
into  a leaden  basin.  (This  is  easily  made  from 
a small  piece  of  sheet  lead.)  Cover  the  fluoride 
with  a little  strong  sulphuric  acid  and  bring  over 
the  top  of  the  basin  a glass  plate  covered  with 
wax,  on  which  a design  has  been  scratched.  On 
melting  the  wax  and  removing  it  from  the  plate 
the  design  will  be  found  to  be  etched  in. 


H 


m 


CHAPTER  X. 


SULPHUR  AND  ITS  COMPOUNDS. 


Classification  of  Sulphur  Compounds. — Some  of 
these  compounds,  such  as  the  sulphites,  thiosul- 
phates, persulphates,  etc.,  are  used  in  photo- 
graphic work  continually,  and  a clear  idea  of 
these  compounds  in  photography  can  only  be  ob- 
tained by  understanding  their  chemical  behavioi*/ 
under  varying  conditions.  In  the  following 
scheme  the  names  of  these  compounds,  together 
with  their  formulae,  are  given,  and  their  relation- 
ship to  sulphur  (see  p.  115). 

Compounds  in  which  the  oxygen  has  been  re- 
placed by  equivalent  quantities  of  sulphur  are 
termed  thio  compounds.  For  example  : — 

NaaSOaO  {i.e.  NasSO^)  = Sodium  sulphate. 
Na^SOgS  {i.e.  Na2S20g)  = Sodium  thiosulphate. 
NH^CNO  = Ammonium  cyanate. 

NH^CNS  = Ammonium  thiocyanaJte. 


'This  nomenclature  is  not  systematically  adhered 
to  in  all  cases,  but  sufficient  has  been  said  to  give 
the  photographer  some  idea  of  the  method 
chemists  use  in  naming  these  more  complicated 
compounds. 

Sulphur. — This  element  is  not  employed  as 
such  in  photography,  but  is  produced  in  many 
bye  operations;  for  example,  when  the  fixing  bath 
becomes  acid.  A very  remarkable  fact  to  be 
noticed  about  sulphur  is  that  it  exists  in  four 
different  physical  forms.  One  variety  occurs  in 
rhombic  prisms,  a second  in  monoclinic  prisms. 


(NH2)oCO 

(NH2>,CS 


— Urea. 

= Thiourea. 


SULPHUR  AND  ITS  COMPOUNDS. 


115 


O QJ 


cJ  ^ 

o "9 

•g.2 


^00 


A 


■§1 

’§  ® 

Si, 

S' I II 


o 


” w 

M rP 

S W 


A 


A 


o a 

m ^ 

§ I 

rri  ’-^ 

p ^ -u 


3 

Pi 

Ph 


o 

p< 

w 


d 

a- 


j;! 

s :3 

Q>  GQ 

I " 

Pii<' 
U1  cp 


■pi 'S 


P< 


o 

I II 

.2 

3 So 


i> 


3 O) 


P 

il 

O « 


Salts  = Mwcyan«2!(?5  or  sulphocyanides. 


116 


PHOTOGRAPPIIC  CHEMISTRY. 


Another  variety  is  plastic,  and  can  be  drawn  out 
in  a similar  manner  to  india-rubber.  The  fourth 
variety  is  non-crystalline  and  non-plastic.  It  is 
in  this  last  modification,  known  as  “ milk  of  sul- 
phur,’^ that  it  is  met  with  in  photography.  Add 
a small  quantity  of  hydrochloric  acid  to  a solu- 
tion of  hypo,  and  notice  the  white  deposit  of 
sulphur  produced. 

Allotropism. — When  an  element  occurs  in  more 
than  one  physical  form,  which  are  each  identical 
chemically,  the  phenomenon  is  termed  one  of 
“ allotropism,’’  and  the  physical  varieties  are 
termed  allotropic  modifications.  As  will  be  seen 
later,  metallic  silver  exists  in  various  allotropic 
forms. 

Sulphur  Dioxide. — When  sulphur  is  burnt  in 
the  air  or  oxygen  it  undergoes  oxidation,  forming 
sulphur  dioxide. 

S + O,  = SO,. 

It  is  better  prepared  in  the  following  manner  ; 
Fit  up  the  apparatus  as  described  under 
“ Chlorine  ” (p.  103).  Into  the  small  flask  place 
some  sodium  sulphite,  and  cover  wfith  a little 
concentrated  hydrochloric  acid.  Warm  cautiously 
and  collect  the  gas  by  downward  displacement. 
The  operation  is  best  carried  out  in  the  open  air, 
care  being  taken  not  to  breathe  the  gas.  Equa- 
tion : — 

Na,S03  + 2HC1  = 2NaCl  + SO,  + H,0 

Sodium  sulphite. 

Note  that  the  gas  has  the  odour  of  burnt  sul- 
phur. Pass  some  of  the  gas  into  water,  and  then 
treat  as  under  with  successive  portions  : — 

{a)  Add  a little  blue  litmus  solution.  Note 
that  it  is  turned  red.  This  is  due  to  the  presence 
of  sulphurous  acid,  which  is  produced  in  accord- 
ance with  the  following  equation  : — 

H,0  + SO,  = H,S03. 

{h)  Introduce  a piece  of  red  paper  into  some 


fetTLPHtJR  AND  ITS  COMPOUNDS. 


11? 


of  the  solution,  or  pass  the  gas  through  some  old 
brown  pyro  solution.  Notice  that  the  red  or 
brown  colour,  as  the  case  may  be,  gradually  dis- 
appears. This  shows  that  sulphur  dioxide  is  a 
reducing  agent.  The  salts  of  sulphurous  acid  are 
the  sulphites. 


Sulphurous  acid.  Sodium  bisulphite.  Sodium  sulphite. 


Tteducing  Action  of  Sulphites. — These  salts  are 
reducing  agents  like  sulphurous  acid.  They 
gradually  oxidise  on  exposure  to  air,  forming 
the  corresponding  sulphates  : — 

NaHS03  + O = NaHSO, 

ISsiSO,  + O = Na,SO, 

When  developing  with  pyrogallol,  dark-coloured 
oxidation  products  are  obtained  which  stain  the 
gelatine.  The  longer  the  development  the  deeper 
is  the  colour.  To  a great  extent  the  presence  of  a 
sulphite  in  the  developing  solution  prevents  the 
formation  of  this  staining.  There  is,  however,  a 
limit  to  the  amount  of  sulphite  which  can  be 
used,  as  a large  excess  of  this  substance  acts  as  a 
restrainer  and  practically  stops  development. 

Sulphur  Dioxide  from  Hypo.— Add  to  a little 
strong  solution  of  hypo  some  hydrochloric  acid. 
Notice  the  presence  of  sulphur  dioxide,  and  ob- 
serve the  deposit  of  sulphur  (milk  of  sulphur). 
The  change  taking  place  in  this  case  is  this  : — 

Na,S203  + 2HC1  - 2NaCl  + H,0  + SO^  + S 

Sodium  tliiosulpbfite. 

(Hypo.) 

Note  especially  that  : — 

(1)  Any  sulphite  treated  with  acid  produces 
sulphur  dioxide. 

(2)  Any  thiosulphate  treated  with  acid  pro- 
duces sulphur  dioxide  and  sulphur. 

Thiosulphates. — These  compounds  may  be  re- 
garded as  sulphates  in  which  an  atom  of  oxygen 


118 


PHOTOGRAPHIC  CHEMISTRY. 


has  been  replaced  by  an  atom  of  sulphur.  They 
are  derivatives  of  a very  unstable  acid  known  as 
thiosulphuric  acid  (H2S0O3).  Sodium  thiosul- 
phate, or  hyposulphite,  commonly  termed  hypo, 
is  obtained  by  boiling  an  aqueous  solution  of 
neutral  sodium  sulphite  with  flowers  of  sulphur. 
Na2S03  -t-  S = Na2S203 

Sodium  sulphite.  P-  Sodium  thiosulphate. 

It  might  be  noted  here  that  the  term  hyposulphite 
for  this  compound  is,  strictly  speaking,  wrong. 
Sodium  hyposulphite  is  the  sodium  salt  of  hypo- 
sulphurous  acid,  H2SO2,  and  would  have  the 
formula  Na2S02.  It  has  very  different  properties 
from  sodium  thiosulphate,  Na2S20a.  Sodium 
thiosulphate  crystallises  with  five  molecules  of 
water  in  large  colourless  prisms.  It  is  somewhat 
deliquescent  if  exposed  to  the  air.  An  iodine 
solution  is  instantly  decolorised  by  a solution  of 
sodium  thiosulphate,  with  the  formation  of  a 
compound  termed  sodium  tetra-thionate  and 
sodium  iodide. 


:>S.O, 


Na 
Na'/ 


Na.SjOe  + 2NaI. 

Sodium 

tetra-thionate. 


Two  molecules  of 
sodium  thiosulphate. 

Fixing  by  Means  of  Thiosulphate. — Sodium 
thiosulphate  has  the  property  of  dissolving  the 
silver  haloids,  and  this  is  taken  advantage  of  in 
removing  the  unaltered  silver  salt  from  the  ex- 
posed negative  or  paper.  They  are  then  said  to 
l3e  “ fixed.’’  If  a concentrated  solution  of  sodium 
thiosulphate  is  added  to  silver  chloride  it  dis- 
solves and  goes  into  solution  as  sodium  silver 
thiosulphate.  This  may  be  represented  as 
follows  : — 

(1)  Ag 
Ag 

Silver  thiosulphate. 


Cl 

+ 

Na 

Cl 

+ 

Na 

\ Ag 

>S,03=  >S,03  + 2NaCl 

Aar 


SULPHUR  AND  ITS  COMPOUNDS. 


119 


(2)  Ag,SA  + Na,SA  = Na,Ag,(SA) 

Sodium  silver 

tliiosuipiiate. 

In  the  combined  toning  and  fixing  bath  it  is  prob- 
able that  the  thiosulphate  plays  an  important 
part  in  the  toning  by  forming  lead  and  gold  thio- 
sulphates, as  well  as  doing  its  duty  as  a fixing 
agent.  The  combined  bath  is  acid  in  its  reaction, 
and  great  care  must  be  taken  to  see  that  the  prints 
are  thoroughly  washed ; it  has  already  been  noted 
that  thiosulphates  in  the  presence  of  acid  gradu- 
ally undergo  decomposition,  producing  sulphur 
dioxide  and  free  sulphur.  These  substances,  of 
course,  bleach,  and  gradually  destroy  the  print. 
Old  baths  containing  thiosulphate  gradually 
undergo  oxidation,  with  the  liberation  of  sulphur, 
etc.,  and  should  not  be  used  for  fixing,  as  they 
will  bleach  or  discolour  the  print  or  negative. 

Removal  of  Thiosulphate. — Various  substances 
have  been  suggested  in  order  to  remove  the  thio- 
sulphate from  the  print  or  negative  in  a quicker 
manner  than  by  washing.  Some  of  these  are  : 
iodine,  bromine  water,  bichromates,  perman- 
ganate, etc.  These  should  be  used  with  caution, 
as  they  more  or  less  attack  the  film  or  image. 
Lumiere  Bros,  and  Seyewitz  recommend  a neutral 
or  alkaline  solution  of  ammonium  persulphate. 
It  is  rendered  neutral,  or  alkaline,  by  adding  to 
it  the  alkaline  bicarbonates,  carbonates,  acetates, 
tungstates,  etc. 

The  F er sulphates. —Hh.Q  free  acid,  persulphuric 
acid  (H2S2O8),  has  not  been  isolated  in  a pure 
condition.  A solution  of  the  acid  is  obtained 
at  the  anode  when  an  electric  current  is  made  to 
pass  through  a well-cooled  solution  of  sulphuric 
acid.  If  an  electric  current  is  passed  through  a 
well-cooled,  concentrated  solution  of  the  sulphates, 
the  persulphates  are  formed  at  the  anode. 

Employment  of  Persulphates  in  Photography. 
— These  persulphates  are  used  in  certain  photo- 


120 


PHOTOGBAPHIC  CHEMISTBY. 


mechanical  processes  where  etching  is  necessary 
and  as  a photographic  “ reducer.’^  They  etch 
metals  by  converting  them  into  sulphates.  For 
instance,  the  action  of  ammonium  persulphate 
on  metallic  zinc  is  as  follows  : — 

(NHJ.S.Os  + Zn  = (NHJ^SO,  + ZnSO, 

Ammonium  Ammonium  ■,  Zinc 

persulphate.  sulphate.  ^ sulphate. 

Ammonium  Persulphate  in  Photographic  Re- 
duction.— It  sometimes  happens  that  a negative 
after  development  is  too  dense,  and  has  to  be 
reduced.  To  bring  this  about,  “ reducers  are 
employed.  The  density  of  an  image  is  due  to  the 
amount  of  metallic  silver  on  the  plate,  and  the 
principle  of  reduction  is  the  gradual  conversion 
of  this  silver  into  some  soluble  salt  so  that  it  can 
be  removed.  Ferric  chloride  in  the  presence  of 
metallic  silver  is  converted  into  ferrous  chloride, 
silver  chloride  being  produced  at  the  same  time, 
which  is  removed  by  sodium  thiosulphate. 

FeClg  + Ag  = FeCL  + AgCl. 

Potassium  ferricyanide  and  sodium  thiosulphate 
also  bring  about  reduction  owing  to  the  formation 
of  silver  ferricyanide,  which  is  soluble  in  the 
thiosulphate.  Lately  ammonium  persulphate  has 
come  into  use  towards  this  end.  This  compound, 
when  used  as  a reducing  agent,  diminishes  con- 
trast— that  is,  it  attacks  the  high  lights,  repre- 
sented by  the  greatest  deposit  of  silver,  in  prefer- 
ence to  the  shade.  The  persulphate  attacks  the 
gelatine  film,  containing  the  most  silver,  convert- 
ing it  into  silver  sulphate,  and  is  itself  reduced 
to  the  ordinary  sulphate,  accompanied  by  the 
formation  of  sulphuric  acid  and  oxygen. 

Ammonium  " Thiocyanate  or  Sulpho cyanide. — ■ 
This  compound  may  be  obtained  by  heating  a solu- 
tion of  ammonium  cyanide  with  sulphur.  It  is 
most  readily  procured  by  heating  carbon  disul- 
phide (an  inflammable  liquid  obtained  by  passing 


SULPHUR  AND  ITS  COMPOUNDS. 


121 


sulphur  vapour  over  heated  carbon)  with  alco- 
holic ammonia. 

2NH,CN  + 2S  = 2NH,CNS. 

Ammonium  cyanide.  = Ammonium  thiocyanate. 

CB,  + 4NH3  = NH,CNS  I-  (NH,),S. 

Carbon  Ammonium  Ammonium 

disulphide.  thiocyanate.  sulphide. 

It  may  be  purified  by  recrystallisation  from  water 
or  alcohol.  It  is  used  in  photography  in  con- 
junction with  gold  chloride  in  the  operation  of 
toning. 

Thiocarh  amide  or  Thio-urea  (CS(NHo)n). — 
When  ammonium  thiocyanate  is  heated  to  about 
110-180°  C.  it  undergoes  a peculiar  intra-molecular 
change  and  passes  into  thio-urea. 


NH.CNS  1>  CS 

Ammonium  NH 

thiocyanate.  Thfocarkmiide. 

This  compound,  thio-urea,  has  the  property,  in 
the  presence  of  a developer,  of  inducing  reversal 
— that  is,  a positive  is  obtained  instead  of  a 
negative. 


122 


CHAPTER  XL 

METALS,  ALKALI  METALS,  ETC. 

Group  of  the  Alkali  Metals. — So  far  only  the 
chemistry  of  the  more  important  non-metals  tak- 
ing part  in  photographic  operations  has  been 
considered,  very  little  mention  being  made  of  the 
metals.  It  is  now  proposed  to  consider,  in  an  ele- 
mentary manner,  a few  important  metals  used, 
in  various  forms,  in  photographic  work.  By 
making  a detailed  study  of  the  metals  it  is  found 
that  they  divide  themselves  into  certain  well- 
characterised  groups.  One  important  group  is 
known  as  the  alkali  metals.  This  group  includes 
potassium,  sodium,  lithium,  rubidium,  and 
caesium.  All  these  are  used,  with  the  exception, 
perhaps,  of  rubidium,  in  photography.  As  a 
rule,  the  non-metal  combined  with  them  is  of 
more  importance  than  the  actual  metal. 

Photographic  Uses  of  Alkali  Metals. — The  bro- 
mides, iodides,  and  chlorides  of  the  alkali  metals 
are  used  in  emulsion  making  and  as  restrainers. 
Their  carbonates  and  sulphites  are  used  in  de- 
velopment, and  thiosulphates  and  cyanides  in 
fixing.  With  regard  to  the  metals  themselves  they 
are  all  very  similar  in  chemical  properties.  They 
are  soft,  easily  fusible  metals,  decomposing  water 
wdth  great  care,  being  at  the  same  time  converted 
into  the  corresponding  hydroxide  or  hydrate. 

Na^  + = 2XaOH  + 

Sodium.  Caustic  soda. 

Sodium  hydroxide. 

All  the  metals  are  monovalent.  Although  the 
alkali  metals  show  a great  similarity  in  chemical 
behaviour,  there  are  one  or  two  points  to  be 


METALS,  ALKALI  METALS,  ETC 


123 


noticed  by  the  photographer.  In  the  first  place, 
potassium  salts  are  more  reactive,  or  stronger  in 
their  chemical  behaviour  than  the  sodium  com- 
pounds, and  secondly  the  potassium  compounds 
in  most  cases  are  more  soluble  in  water  than  the 
corresponding  salts  of  sodium. 

The  Metals  of  the  Alkaline  Earths. — Three  com- 
mon metals  constitute  this  group,  barium,  stron- 
tium, and  calcium,  and  they  manifest  a very 
similar  chemical  behaviour.  The  metals  them- 
selves are  isolated  with  difficulty,  and,  like  the 
alkali  metals,  decompose  water  readily  with  the 
formation  of  the  hydrate. 

20a  + 4H,0  = 2Ca  + 2H, 

Calcium  hydrate. 

Here,  again,  in  most  cases  it  is  the  acid  or  non- 
metal  in  combination  with  the  barium,  strontium, 
or  calcium,  which  is  important  in  photography. 

Calcium  G onvpoands. — Calcium  carbonate  or 
chalk  (CaCOg)  is  often  used  for  neutralising  acids. 

2HC1  + CaC03  = CaCL  + H,0  -f  CO^ 

The  calcium  haloids  are  used  in  the  preparation 
of  certain  emulsions.  They  react  with  the  silver 
nitrate  in  the  same  manner  as  the  potassium 
or  sodium  haloids. 

SAgNOg  + Cal2  = 2AgI  -f-  Ca(N0s)2 

Calcium  iodide.  Calcium  nitrate. 

The  fused  calcium  chloride,  owing  to  its  pro- 
perty of  absorbing  water,  is  used  as  a drying 
agent  in  the  storing  of  platinum  paper.  Bleach- 
ing powder,  having  the  formula  CaOCL  (calcium 
chlorohypochlorite)  is  used  in  photographic  re- 
duction. 

Barium  and  Strontium  C ompounds. — The 
haloid  compounds  of  these  metals  are  also  used  in 
emulsion  making. 

SrCl^  + 2AgN03  - 2AgCl  + Sr(N03)2 

Strontium  chloride.  Strontium  nitrate. 


124 


MoTOGRAPmC  chemistry. 


Magnesium  Group. — Four  metals  are  met  with 
here — magnesium,  zinc,  cadmium,  and  mercury. 
They  are  very  similar  in  most  of  their  chemical 
reactions,  but  do  not  show  such  a complete  anal- 
ogy as  the  alkali  metals.  Magnesium  is  exten- 
sively used  in  the  preparatioij  of  flash-light  mix- 
tures. These,  as  a rule,  consist  of  magnesium  and 
finely-divided  aluminium,  with  potassium  chlor- 
ate, or  some  other  body  rich  in  oxygen.  The 
“ flash  ” or  light  given  out  by  the  burning  mag- 
nesium is  rich  in  chemically  active  rays.  The 
equation  is  : — 

2Mg  -f-  O2  = 2MgO 

Magnesium.  Magnesium  oxide. 

Great  care  should  be  exercised  when  working  with 
these  “ flash-light  ” mixtures.  The  zinc  and  cad- 
mium haloids  are  used  in  emulsion  making. 

Znlj  + 2AgN03  = 2AgI  -f  Zn(N03)2 

Zinc  iodide.  Zinc  nitrate. 

CdBr2  + 2AgN03  = 2AgBr  + 

Cadmium  Cadmium 

bromide.  nitrate. 

Mercury. — This  is  the  only  metal  which  is  a 
liquid  at  ordinary  temperatures.  The  vapours  of 
this  metal  were  used  for  developing  the  Daguer- 
reotype image.  Silver  plates  previously  treated 
with  the  vapours  of  a halogen  were  exposed  to 
light.  On  treatment  with  mercury  vapour,  the 
vapour  condensed  on  the  parts  exposed  to  the 
light  and  produced  a visible  image.  Mercury 
dissolves  almost  all  metals  (not  iron),  and  the 
resulting  products  are  termed  amalgams.  In  the 
intensification  process,  using  mercuric  chloride 
and  then  ferrous  oxalate,  an  amalgam  of  silver  is 
produced  in  the  final  reaction. 

Mercury  Haloids. — Mention  has  already  been 
made  of  the  fact  that  mercury  forms  two  chlorides 
— the  mercurous  and  mercuric  compounds.  These 
are  the  chief  mercury  salts  used  in  photography. 


METALS,  ALKALI  METALS,  ETC.  125 


Mercuric  Chloride  (HgCL).  Corrosive  sub- 
limate.— This  compound  is  obtained  on  the  large 
scale  by  subliming  a mixture  of  mercuric  sulphate 
and  sodium  chloride. 

HgSO,  + 2NaCl  = HgCl,  + NaSO, 

Mercuric  sulphate.  Mercuric  chloride. 

It  is  not  very  soluble  in  water,  but  is  more  soluble 
in  alcohol. 

Mercuric  chloride  also  forms  double  chlorides 
with  many  metallic  chlorides. 

HgCl^  + NaCl  - HgNaCl3 
With  ammonia  a bulky  white  precipitate  is  pro- 
duced of  varying  composition. 

Action  of  Silver  on  Mercuric  Chloride. — 
Metallic  silver,  when  treated  with  mercuric 
chloride  solutions,  abstracts  an  atom  of  chlorine 
with  the  production  of  argentic  mercurous 
chloride  : 

Ag  -f  HgCL  = AgHgCl^ 

Argentic  mercurous 
chloride. 


This  fact  is  taken  advantage  of  in  intensification, 
the  AgHgCla  compound  being  referred  to  as  the 
“ bleached  image.” 

Action  of  Light  on  IlgCU^. — A solution  of  mer- 
curic chloride  undergoes  photo-reduction  on  ex- 
posure to  light,  with  the  separation  of  mercurous 
chloride.  This  change  may  be  represented  in  the 
following  manner  : — 


H 

H 

>0 

= 2HgCl 

Mercurous 

^°\C1 

Mercuric 

chloride. 

chloride. 

Copper,  Silver,  and  Gold. — This  group,  con- 
taining, as  it  does,  silver  and  gold,  is  the  most 
important  one  to  the  photographer,  and  is  there- 
fore dealt  with  more  fully  than  the  preceding 
groups. 


126  PHOTOGRAPHIC  CHEMISTRY. 


Copper. — This  metal  is  not  of  great  import- 
ance in  photographic  operations.  Cupric  salts 
undergo  photo-reduction  in  the  presence  of  light, 
the  action  being  assisted  by  the  presence  of  oxidis- 
able  compounds,  as  in  the  case  of  mercuric  salts. 
Thus  cupric  chloride  is  reduced  to  cuprot4S 
chloride. 


/Cl 

Nci  ^ H 
Ici  + H 
Cl 

Cupric  chloride. 


O = 2CuCl  + 2HC1  + O 

Cuproiii-  chloride. 


This  cupric  chloride,  in  conjunction  with  common 
salt,  was  proposed  by  J.  Spiller  in  1883  as  a 
means  of  reducing  dense  negatives.  The  silver  on 
the  plate  reacts  with  the  cupric  chloride  as  fol- 
lows : — 

CuCla  + Ag  = CuCl  -1-  AgCl 

On  plate. 

The  salt  then  removes  the  silver  chloride  as  fast  as 
it  is  formed. 

Silver. — This  is  an  extremely  lustrous  metal, 
crystallising  in  cubes  or  eight-sided  crystals. 
When  obtained  from  its  compounds  by  means  of 
nascent  hydrogen  it  appears  as  a grey  powder. 
In  very  thin  sheets  it  is  blue  when  viewed  by 
transmitted  light.  Molten  silver  absorbs  about 
twenty  times  its  volume  of  oxygen.  On  cooling, 
this  gas  is  liberated  almost  completely;  the  silver 
still  retains  a small  quantity  of  oxygen  even  at 
ordinary  temperatures.  Silver  is  harder  than 
gold,  softer  than  copper,  and  in  its  chemical  be- 
haviour it  is  very  similar  to  these  metals. 

Allotropic  Modifications  of  Silver. — In  addi- 
tion to  the  ordinary  form  of  white  metallic  silver 
there  are  several  other  varieties,  which  differ  in 
a very  marked  manner  as  regards  appearance, 
their  action  as  regards  light,  and  solubility  in 
water.  They  were  discovered  by  Carey  Lea,  who 


METALS,  ALKALI  METALS,  ETC. 


127 


carried  out  a series  of  experiments  on  the  proper- 
ties of  silver  precipitated  from  its  solutions  by 
means  of  reducing  agents,  such  as  ferrous  citrate, 
ferrous  tartrate,  and  dextrin,  in  the  presence  of 
an  alkali.  The  precipitates  of  silver  show  almost 
every  shade  of  colour,  from  blue  to  red,  green, 
purple,  and  golden. 

Action  of  Nitric  Acid  on  Silver. — Silver 
readily  dissolves  in  dilute  nitric  acid,  with  the 
formation  of  silver  nitrate  and  the  evolution  of 
oxides  of  nitrogen.  It  crystallises  in  rhombic 
tables  isomorphous  with  potassium  nitrate.  When 
perfectly  pure,  it  is  unaffected  by  light.  Organic 
substances  turn  it  black  owing  to  the  production 
of  silver. 

Action  of  Other  Acids  on  Silver. — Dilute  sul- 
phuric acid  has  practically  no  action  on  silver. 
Hot  concentrated  sulphuric  acid  produces  silver 
sulphate. 

2H,S04  + 2Ag  = Ag,SO,  + SO,  4-  2H,0 

Silver 

sulphate. 

Hydrochloric  acid  has  a very  slight  action  on  the 
metal. 

Hydrobromic  and  hydriodic  acids  attack  sil- 
ver, with  the  production  of  the  corresponding 
silver  haloids  accompanied  by  the  evolution  of 
hydrogen. 

2HI  + Ago  = 2AgI  + H2 
2HBr  + Ago  = 2AgBr  + H2 

Action  of  Potassium  Iodide  and  Cyanide  on 
Silver. — A solution  of  potassium  iodide  in  the 
presence  of  air  slowly  dissolves  silver  with  the 
formation  of  potassio-silver-iodide  (AgKIo).  This 
may  be  represented  by  an  equation  as  follows  : — 

4KI  H-  O + H2O  + 2Ag  = 2KOH  + 2AgKl2 
Hot  potassium  cyanide  solution  acts  on  metallic 
silver  to  produce  a solution  of  potassio-silver- 
cyanide  as  follows  : — 


128 


PHOTOGRAPHIC  CHEMISTRY. 


4KCN  + 2HOH  + 2Ag  = 2KOH  + 2AgK(CN>2 

+ H,. 

Silver  Haloids. — Silver  Chloride  {AgCl). — 
This  compound  is  produced  when  any  soluble 
chloride,  such  as  those  of  sodium,  cadmium,  am- 
monium, zinc,  lithium,  etc.,  is  added  to  a solu- 
tion of  silver  nitrate.  Attention  has  been  already 
drawn  to  this  in  the  preceding  articles.  Stas  dis- 
tinguishes four  allotropic  varieties,  two  common 
forms  being  the  flocculent  and  the  powdery.  It 
is  soluble  to  a slight  extent  in  boiling  water  and 
concentrated  hydrochloric  acid.  It  is  also  solu- 
ble, to  a far  greater  extent,  in  concentrated  solu- 
tions of  the  chlorides  of  Ba,  Sr,  Ca,  Mg,  Na,  and 
K.  One  litre  of  aqueous  ammonia  of  sp.  gr. 
0’924  dissolves  69*5  grammes  of  silver  chloride. 

2NH,0H  + AgCl  = AgCl2NH3 

Soluble. 

Dry  silver  chloride  absorbs  ammonia  gas  to 
form  a white  compound  having  the  formula 
2AgCl3NH3.  Potassium  cyanide  readily  dissolves 
silver  chloride,  forming  potassio-silver-cyanide. 

AgCl  + 2KCN  = AgK(CN)2  + KOI. 

The  best  solvent  for  silver  chloride  is  undoubtedly 
sodium  thiosulphate.  Mercuric  nitrate  and  silver 
nitrate  in  concentrated  solution  also  dissolve  sil- 
ver chloride. 

Silver  Bromide  (AgBr). — This  compound  is 
produced  by  treating  silver  nitrate  solution  with 
any  soluble  bromide.  Stas  describes  six  allotropic 
modifications. 

Silver  bromide  dissolves  in  most  of  the  sol- 
vents mentioned  under  silver  chloride.  It  dis- 
solves, however,  with  more  difficulty  in  ammonia 
than  in  the  chloride.  In  other  respects  it  is  per- 
fectly similar  to  the  latter. 

Silver  Iodide  (Agl). — Like  the  chloride  and 
bromide,  this  compound  is  obtained  by  adding  any 


J^IETALS,  ALKALI  METALS,  ETC. 


129 


soluble  iodide  to  a solution  of  silver  nitrate.  Two 
forms  of  the  iodide  apparently  exist.  One  variety, 
obtained  by  precipitating  an  alkaline  iodide  with 
an  excess  of  silver  nitrate  solution,  is  of  a curdy 
yellow  appearance,  and  the  other  is  produced  as 
a yellow  powder  by  adding  excess  of  alkaline 
iodide  solution  to  silver  nitrate.  This  last  is 
more  sensitive  to  light  than  the  curdy  yellow 
modification.  Silver  iodide  is  distinguished  from 
the  chloride  and  bromide  by  its  yellowish-green 
colour  and  its  insolubility  in  ammonia.  In  other 
respects  it  behaves  like  the  chloride.  It  is  soluble 
in  silver  nitrate  solution,  provided  it  contains 
more  than  3 per  cent.  AgNOg.  On  the  addition 
of  water  the  iodide  is  reprecipitated. 

Gold. — This  metal,  in  the  form  of  chloride  or 
in  combination  with  chlorine  and  sodium  chloride 
as  sodium-chloraurate,  is  used  in  toning.  It  ap- 
parently exists  in  three  allotropic  modifications. 
One  variety  is  ordinary  gold  of  a bright  yellow 
lustre,  and  if  beaten  out  into  thin  sheets  (gold 
leaf)  is  green  by  transmitted  light.  The  second 
variety  is  obtained  as  a dark  brown  powder  when 
many  reducing  agents,  such  as  ferrous  sulphate, 
oxalic  acid,  etc.,  are  added  to  solutions  of  gold. 
The  third  variety  is  soluble  in  water,  and  is  ob- 
tained in  a rather  complicated  manner  hy  what  is 
known  as  Heinrich’s  method.  Gold  salts  may 
easily  he  recognised  by  the  hrown  precipitate  of 
metallic  gold  which  is  thrown  down  when  ferrous 
salts  are  added. 

Two  Classes  of  Gold  Salts. — Gold  exists  in  two 
conditions  in  its  comi3ounds;  as  aurous,  in  which 
it  is  monovalent,  and  as  auric,  in  which  it  is 
trivalent.  The  auric  compounds  are  used  in 
photography.  The  metal  is  soluble  in  aqua  regia, 
and  solutions  containing  free  chlorine  (see  “ Gold 
Chloride,”  p.  111).  The  chloride  has  an  acid  re- 
action, and  has  the  composition  HAuCl^,  hence 
it  is  termed  chlorauric  acid[. 

I 


130 


PHOTOGRAPHIC  CHEMISTRY. 


H K NH^  Na 

II  I I 

Au  Au  + 2H2O  Au  + 2IH2O  Au  + 2H2O 

II  I i 

cu  Cl,  Cl,  Cl, 

y . " f ' Y ' 

Chlor-  Potassium  Ammonium  Sodium 

auric  acid.  cliloiaurate.  cliloraurate.  chloraurate. 

Aurates. — Auric  acid  (HAuOa).  By  heating 
auric  chloride  (A11CI3)  with  magnesia,  and  then 
treating  with  dilute  nitric  acid  to  remove  excess 
of  Mg,  auric  oxide  is  obtained  as  follows  : — 

2AUCI3  + 3MgO  = AU2O3  + SMgCla 
This  auric  oxide  dissolves  in  caustic  potash  to 
form  potassium  aurate,  which  crystallises  in 
bright  yellow  needles. 

AU0O3  + 2KOH  = 2KAUO2  + H2O 

Potassium  aurate. 

On  the  addition  of  silver  nitrate  solution  to  this 
potassium  aurate  a precipitate  of  silver  aurate 
is  obtained. 

KAuOa  + AglSrOg  = AgAu02  + KNO3 

Silver  aurate. 

Uranium. — Uranium  is  used  in  photographic 
work  for  toning  broihide  prints  and  in  the 
uranium  intensifying  process.  With  regard  to  the 
chemistry  of  this  metal,  all  that  the  photographer 
need  notice  is  that,  in  the  first  place,  it  has  a 
valency  of  four  in  uranous  compounds  and  a val- 
ency of  six  in  uranic  compounds.  Secondly,  the 
oxide  UO2,  termed  uranyl,  takes  the  place  of  the 
simple  metal  uranium  in  its  salts.  Thus  : 

U02(N03)2  UO2SO, 

Urauyl  nitrate.  Uranyl  sulphate. 

Platinum- — This  metal,  like  gold,  is  not  at- 
tacked by  acids;  it  is  only  soluble  in  liquids 
generating  free  chlorine,  such  as  aqua  regia.  It 
forms  two  classes  of  salts;  in  the  platinous  com- 
pounds it  is  divalent,  and  in  the  platinic  tetra- 
valent. 


METALS,  ALKALI  METALS,  ETC. 


131 


PtCl,  PtCl, 

Platinous  chlor'de.  Platinic  chloride. 

Platinic  Chloride  (PtCl^)  is  obtained  when 
platinum  is  dissolved  in  aqua  regia.  On  evapora- 
tion the  chloroplatinic  acid  separates  in  brownish- 
red  deliquescent  crystals,  containing  six  molecules 
of  water.  It  forms  characteristic  double  chlorides 
with  ammonium  and  potassium,  which  are  only 
soluble  in  water' with  difficulty. 

PtCl,  + 2HC1  - H^PtCle 

Cliloroplatinic  acid. 

PtCl,  + 2KC1  = K.PtCl^ 

Potassium  chloroi>latinate. 

Platinous  Chloride  (PtCL). — By  cautiously 
heating  platinic  chloride  to  200°  to  250°  C.,  it  loses 
a molecule  of  chlorine  and  is  converted  into  solu- 
ble platinous  chloride,  a greenish  powder. 

PtCl,  = PtCl^  -f  CL 

Platinic  chloride.  Platinous  chloride. 


With  hydrochloric  acid  the  platinous  chloride 
forms  easily  soluble  chloroplatinous  acid. 

PtCL  + 2HC1  = H,PtCl, 

Chloroplatinous  acid. 

With  the  alkaline  chlorides  it  forms  easily  solu- 
ble chloroplatinites  : 


PtCL  + 2KC1  = K.PtCL 

Potassium  chloroplatinite. 

The  photographer  will  see  the  connection  better, 
perhaps,  between  the  various  compounds  men- 
tioned above  if  they  are  compared  as  under  : 


PtCL  — ^ H.PtCl,  — 1>  K.PtCL 

Platinic  {>  Chhjroplatinic  p Potassium 

chloride.  acid.  chloroplatinate. 

V 


PtCl,  — > H.PtCl,  — > KjPtCI. 

Platinous  P Chloro]datinous  Potassium 

chloride.  acid.  chloroplatinite. 

Platinotype. — The  salt  used  in  platinotype 
printing  is  potassium  chloroplatinite.  In  prac- 


132 


PHOTOGKAPHIC  CHEMISTRY. 


tice  it  is  conveniently  prepared  by  reducing  the 
chloroplatin«^6  with  freshly  precipitated  cuprous 
chloride. 

K^PtCle  + CU2CI2  = K^PtCl,  + CuCl^ 

Potassium  + Cuprows  > Potassium  + Cupric 

chloroplatinate.  cliloride.  chloroplatinite.  chloride. 

It  is  essential  to  the  proper  working  of  platinum 
paper  that  it  should  be  kept  dry,  and  towards  this 
end  fused  calcium  chloride  is  employed  in  the 
tins  used  for  storing  it. 

The  Alums. — These  compounds,  of  which  several 
are  used  in  photographic  work,  are  obtained  by 
crystallising  together  the  sulphates  of  aluminium, 
chromium,  and  iron  with  those  of  the  alkali 
metals,  Na,  K,  etc. 

The  ordinary,  or  potash,  alum,  has  the  follow- 
ing formula  : 

K,SO,  + AIJSOJ3  + 24H3O 

Potassium  , Aluminium  , Water  of 

sulphate.  sulphate.  cryst  illisation. 

It  is  important  to  remember  that  this  substance 
has  an  acid  reaction.  It  is  principally  used  for 
hardening  the  surface  of  gelatine  films. 

The  connection  between  potash  alum  and  the 
other  alums  is  readily  seen  by  examining  the 
following  formulae  : — 


Aluminium  Alums  : 

KoSO^  + A1o(S04)3  + 24H2O  Potash  alum. 
Na2  SO4  + ALCSO^^a  + 24H2O  Soda  alum. 
(NH4)2S04  + ALCSO^)^  + 24H2O  Ammonia  alum. 

Chromium  Alums  : 

K,SO.  + Cr,(SO,),  + 24H,OP— 
Na,SO,  + Cr,(SO,>3  + 24H3O  „ 
(NHJ^SO,  + Cr3(SOj3  + 24H30^™omralum. 
Iron  Alums  : 

K3SO.  + Fe3(S03>3  + 24H30>^?“Z. 
Na^SO,  + Fe3(S03>3  + 24H3O 
(NHJ3SO.  + Fe3(S03>3  + 24H30^SKum.' 


^METALS,  ALKALI  METALS,  ETC.  133 

The  Cyanides. — Compounds  containing  the 
group  (CN)  are  numerous  and  important.  The 
chief  compounds,  from  a photographic  point  of 
view,  are  potassium  cyanide  and  the  ferro-  and 
ferri-cyanides. 

. Potassium  Cyanide  (KCN).— This  substance  is 
the  potassium  salt  of  prussic  or  hydrocyanic  acid 
(HCN).  The  cyanide  forms  double  salts  with 
metals,  which  are,  as  a rule,  very  soluble  in  water. 
Owing  to  this  fact,  potassium  cyanide  is  used  as  a 
fixing  agent.  It  readily  dissolves  the  silver 
haloids  forming  potassio-silver-cyanide.  The 
equations  representing  the  change  are  : 

2KCN  + AgCl  = AgK(CN)2  + KOI. 

2KCN  H-  AgBr  = AgK(CN).  + KBr. 

2KCN  + Agl  = AgK(CN)^  + KI. 

Potassio-silver 

cyanide. 

Ferro  cyanides  and  Ferricyanides. — These  com- 
pounds are  employed  in  certain  processes  for 
bringing  about  the  reduction  or  intensification  of 
negatives,  and  for  the  preparations  of  blue  prints. 
Potassium  ferrocyanide  or  yellow  prussiate  of 

potash  has  the  formula  K4Fe(CN)6. 

If  added  to  a solution  containing  a ferric  salt 
a dark  blue  precipitate  of  Prussian  blue  is  ob- 
tained. 

3K4Fe"(CN)3  + 4FeCl3  = Fe/[Fe'"(CN)  J3  -t- 

Potassinm  Ferric  Prussian  blue  12KC1 

ferrocyanide.  chloride.  or  ^ 

Ferric  ferrocyanide. 

In  the  presence  of  an  oxidising  agent,  such  as 
chlorine,  potassium  ferrocyanide  is  converted  into 
potassium  ferricyanide  or  red  prussiate  of  potash. 
Conversely,  potassium  ferricyanide  in  the  presence 
of  organic  matter,  especially  under  the  influence 
of  light,  is  reduced  to  the  ferrocyanide. 


134 


CHAPTER  XII. 

ORGANIC  OR  CARBON  COMPOUNDS  USED  IN 
PHOTOGRAPHY. 

Meaning  of  “ Organic^ — Perhaps  the  most  im- 
portant compounds  used  in  photography  are  the 
so-called  “ organic  ” developers,  such  as  pyro- 
gallol,  amidol,  eikonogen,  etc.  Before  discussing 
these  substances  it  would  be  as  well  to  have  some 
idea  of  the  meaning  of  the  term  “ organic.”  In 
the  early  days  of  chemistry  two  classes  of  chemi- 
cal compounds  were  recognised,  those  obtained 
from  mineral  substances  and  those  of  animal  or 
vegetable  origin.  It  was  held,  at  that  time,  that 
substances  of  the  latter  character  could  not  be 
obtained  artificially. 

Organic  Compounds  Obtained  Artificially. — 
In  1826,  however,  Hennell  obtained  ethyl  alcohol 
artificially,  and  in  1828  Wohler  synthesised  an 
essentially  organic  substance,  urea.  Neither  of 
these  investigators  utilised  the  living  organism 
for  the  production  of  those  compounds.  The  terms 
‘‘  organic  ” and  “ inorganic  ” are,  however,  still 
retained,  simply  for  the  sake  of  convenience.  All 
organic  compounds  contain  the  element  carbon, 
consequently  organic  chemistry  is  often  defined 
as  the  chemistry  of  the  carbon  compounds. 

Classification  of  Carbon  C ompounds. — As  the 
result  of  a detailed  study  of  organic  compounds, 
they  are  found  to  fall  naturally  into  two  groups. 
One  division  contains  those  whose  properties  and 
reactions  can  only  be  explained  by  assuming  that 
the  carbon  atoms  in  the  molecule  are  arranged  in 
the  following  manner  : — 


ORGANIC  OR  CARBON  COMPOUNDS. 


lo5 


! I I i 

_c— C— C— C—  etc. 

I I I I 

As  some  of  the  most  important  members  of  this 
group  were  found  in  various  fats  and  oils,  they 
were  termed  “ fatty  compounds.”  A better  way 
of  referring  to  them,  however,  is  to  call  them 
“ open  chain  ” compounds,  owing  to  the  manner 
in  which  their  carbon  atomiS  are  arranged  in  the 
molecule.  The  second  group  behave  in  a different 
way  towards  chemical  reagents,  and  this  differ- 
ence of  behaviour  can  only  be  accounted  for  on 
the  assumption  that  the  carbon  atoms  in  the  mole- 
cule are  arranged  in  the  form  of  a closed  ring, 
or  cycle,  as  below  : 


-C~C-^ 
I I 
C-C- 
I I 


\ / 


\ 


c 


/ \ / ^ 
c 

/ \ 


Hence  they  are  termed  “ closed  chain  ” or 
cyclic  bodies.  They  are  also  known  by  the  name 
of  “ aromatic  ” compounds,  because  at  one  time 
their  most  characteristic  substances  were  obtained 
from  the  various  aromatic  gums  and  balsams. 

Molecular  and  Constitutional  Formula:. — By 
making  a qualitative  examination  of  the  organic 
compounds  their  component  elements  are  ascer- 
tained, and  those  usually  present  are  found  to  be 
carbon,  hydrogen,  oxygen,  nitrogen,  and  sulphur. 
If  the  compound  is  submitted  to  a quantitative 
analysis,  the  percentage  of  each  element  is  ob- 
tained; and  if  its  physical  properties  in  the  state 
of  vapour  are  examined,  the  chemist  is  then  in 
a position  to  state  the  number  of  atoms  of  each 
element  present  in  the  molecule.  [For  a detailed 


138 


PHOTOGRAPHIC  CHEMISTRY. 


explanation  of  these  processes  the  photographer 
is  referred  to  any  text-book  on  organic  chemistry.] 
By  writing  the  symbols  of  these  elements,  together 
with  their  proper  exponents,  the  molecular  for- 
mula is  obtained.  Thus  : Ethyl  alcohol  contains 
the  elements  carbon,  hydrogen,  and  oxygen. 
From  a quantitative  and  physical  examination  of 
the  substance  it  is  found  to  contain  in  the  mole- 
cule two  atoms  of  carbon,  six  atoms  of  hydrogen, 
and  one  atom  of  oxygen ; its  molecular  formula  is, 
therefore,  CaHgO.  The  constitutional  formula  of 
ethyl  alcohol  will  be  found  on  p.  139. 

Isomerism. — It  so  happens  that  a large  number 
of  organic  compounds  have  the  same  molecular 
formulae,  but  have  different  chemical  and  physical 
properties.  This  peculiarity  is  termed  isomerism, 
and  the  compounds  are  said  to  be  isomeric.  Thus 
the  formula  CgHgOa  represents  three  important 
compounds,  pyrocatechol,  resorcinol,  and  hydro- 
quinone.  The  formula  CaHgO  represents  ethyl 
alcohol  and  methyl  ether.  By  making  a careful 
study  of  the  reactions  and  transpositions  of  iso- 
meric substances  they  are  found  to  differ  in  chemi- 
cal deportment,  which  leads  to  the  assumption 
that  their  molecules  are  differently  arranged  or 
constituted.  A formula  which  shows  this  arrange- 
ment of  the  atoms  in  the  molecule  is  termed  a 
constitutional  formula,  and  a knowledge  of  the 
latter  is  obviously  highly  important  in  dealing 
with  organic  compounds. 

“ Ox>en  Chain  G ompounds. — It  is  now  pro- 

posed to  consider  a few  “ open  chain  ’’  compounds 
used  in  photographic  work.  With  regard  to  the 
hydrocarbons,  the  only  one,  perhaps,  that  the 
photographer  will  have  to  deal  with  is  acetylene, 
which  is  used  for  illuminating  purposes.  This 
hydrocarbon  belongs  to  a series  of  compounds 
having  the  general  formula  CnH2n — 2,  wherein 
is  the  number  of  carbon  atoms.  It  is  obtained 
by  treating  calcium  carbide  with  water. 


ORGANIC  OR  CARBON  COMPOUNDS. 


137 


CaCs  + 2H2O  = Ca(OH)2  + 

Calcium  Calcium  Acetylene, 

carbide,  hydroxide. 

It  will  suffice  to  mention  here  that  it  is  poison- 
ous, and  forms  a highly  explosive  mixture  with 
air. 

The  Alcohol  Family. — Methyl  alcohol,  CH3OH. 
This  compound  is  used  as  a solvent  for  varnish 
making,  etc.  In  the  presence  of  an  oxidising 
agent  it  undergoes  oxidation,  producing  in  the 
first  place  formaldehyde  and  then  formic  acid. 


H 

I 

H— C-O’H  + 0; 

I i i 
:h i 

■ H 

I 

H-C  = 0 + O = 


H 


H-C-0  + H,0 


Formaldehyde, 

OH 

I 

H— 0 = 0 or  HCOOH 

Formic  acid. 


Ethyl  Alcohol,  C2H5OH. — This  is  the  next  al- 
cohol in  the  series  and  has  a variety  of  uses  in 
photography,  principally  as  a solvent.  When 
mixed  with  methyl  alcohol  (commercially  known 
as  wood  spirit)  it  is  termed  methylated  spirit. 
Submitted  to  oxidising  agents  it  passes  first  to 
acetaldehyde,  the  next  aldehyde  to  formaldehyde, 
and  then  to  acetic  acid,  the  next  acid  in  the  series 
to  formic  acid.  If  ethyl  alcohol  is  treated  with 
strong  sulphuric  acid  and  then  heated  to  about 
147°  C.,  a molecule  of  water  is  abstracted  from 
two  molecules  of  the  alcohol,  and  ether  produced. 

Aldehyde  Family. — Formaldehyde,  or  formalin 
HCHO.  Formaldehyde  is  obtained  by  oxidising 
methyl  alcohol,  as  already  mentioned.  It  is  used 
in  photography  for  hardening  the  gelatine  films, 
so  as  to  prevent  frilling  in  hot  weather,  and  also 
as  a preservative  of  mountants,  as  it  destroys 
bacteria.  At  ordinary  temperature  the  formalde- 


138 


PHOTOGRAPHIC  CHEMISTRY. 


hyde  is  a gas,  and  it  is  usually  met  with  in  prac- 
tice as  a 40  per  cent,  solution  under  the  name  of 
formalin. 

Polymerism. — x\fter  standing,  the  formalin 
undergoes  a very  remarkable  change,  producing  a 
variety  of  compounds  which,  on  analysis,  are 
found  to  be  multiples  of  HCHO. 

HCHO  [HCHO],  [HCHO], 

Formaldehyde.  Paraformaldehyde.  Metaformaldehyde. 

Compounds  which  condense  with  themselves  to 
produce  new  compounds,  as  in  the  case  of  form- 
aldehyde, are  said  to  undergo  polymerisation,  and 
the  new  compounds  formed  are  termed  polymers 
of  the  original  substance.  If  exposed  to  the  air 
it  undergoes  oxidation,  producing  formic  acid. 

H OH 

I I 

,HC  = 0 -f  ,0  = ,H-C  = 0 

Formic  acid. 

Owing  to  this  fact,  formaldehyde  and  the  alde^ 
hydes  as  a class  are  powerful  reducing  agents. 
This  reducing  action  is  readily  seen  by  adding 
formalin  to  an  ammoniacal  solution  of  silver 
nitrate.  After  a short  time  silver  separates  on 
the  sides  of  the  vessel  as  a brilliant  mirror. 

Acetaldehyde,  CH,CHO. — This  is  the  aldehyde 
of  acetic  acid,  and  may  be  obtained  by  adding  an 
oxidising  agent  to  ethyl  alcohol. 

C,H,OH  + O = CH3CHO  -I-  H,0 

Ethyl  alcohol.  ^ Acetaldehyde. 

The  aldehyde  combines  directly  with  prussic  acid. 
HCN,  and  the  alkaline  bisulphites;  with  HCN 
they  produce  compounds  termed  cyanhydrins. 

Family  of  Organic  Acids. — The  organic  acids 
form  a very  large  group  of  compounds,  and  for 
purposes  of  study  they  are  divided  into  various 
sub-groups,  according  to  the  radicals  present. 
For  instance,  acids  containing  one  carboxyl  group 
are  termed  monocarboxylic  acids,  those  containing 
two  are  called  dicarboxylic  acids,  and  so  on.  If 


ORGANIC  OR  CARBON  COMPOUNDS. 


139 


the  acid  contains  hydroxyl  groups  as  well,  they 
are  said  to  be  hydroxy-carboxylic  acids,  mono,  di, 
or  tri,  etc.,  as  the  case  may  be.  The  most  common 
organic  acids  used  in  photography  are  probably 
acetic,  oxalic,  and  citric  acids,  together  with  their 
salts. 

Mono-Carhoxylic  Acid  Family. — Acetic  acid, 
CH3COOH.  This  is  a very  common  acid,  and  in 
a dilute  solution,  together  with  colouring  matter, 
it  constitutes  vinegar.  Ordinary  brown  vinegar 
is  obtained  by  allowing  sour  beer,  etc.,  to  undergo 
bacterial  oxidation.  If  spirits  or  white  wines 
are  used  in  place  of  the  beer,  white  vinegar  is 
obtained.  As  is  well  known,  beer  and  spirits 
contain  ethyl  alcohol,  and  when  this  compound 
undergoes  oxidation  it  produces  acetic  acid.  It 
it  due  to  this  acid  that  vinegar  has  a sharp  taste. 

CHg  CHg  CH3 

' ITTIT  —>  H-C  -f-  O — {>  C— OH 

II  II 

H 0 0 

C2H5OH  — 1>  CH3CHO  — i>  CH,,COOH 

Etliyl  alcohol.  Acetaldehyde.  Aeetic  acid. 

The  pure,  concentrated  acetic  acid  is  known  under 
the  name  of  glacial  acetic  acid,  it  being  this 
variety  that  is  principally  used  in  photography. 
Acetic  acid  is  used  with  ferrous  sulphate  in  de- 
veloping wet  collodion  plates.  The  acid  acts  as 
a restrainer,  by  preventing  the  too  rapid  deposi- 
tion of  silver.  It  dissolves  the  silver,  forming 
silver  acetate. 

2CH3COOH  + 2Ag  - 2CH3COOAg  -f  H3 

Silver  acetate. 

This  acid  is  also  employed  in  the  lead  and  uran- 
ium intensifiers  in  order  to  keep  these  solutions 
weakly  acid,  and  for  washing  bromide  prints 
after  development  with  ferrous  oxalate,  or  toning 
with  uranium  salts.  It  should  contain  no  fur- 


H-Gi 

I 

O 


140 


PHOTOGRAPHIC  CHEMISTRY. 


furol  or  formic  acid,  as  these  substances  are  harm> 
ful  for  photographic  purposes. 

Test  for  Furfural. — This  is  a cyclic  body  con- 
taining oxygen,  and  causes  complications  by  act- 
ing as  a reducing  agent.  It  may  be  detected  in 
minute  quantities  by  adding  a drop  of  aniline 
to  the  suspected  acetic  acid.  If  present,  a deep 
red  colour  is  produced,  disappearing  on  standing. 

Test  for  Formic  Acid. — This  acid  acts  as  a 
powerful  reducing  agent.  It  is  detected  by  add- 
ing a solution  of  silver  nitrate.  A brown  precipi- 
tate of  reduced  silver  shows  that  formic  acid  is 
present.  In  some  cases  acetic  acid  is  adulterated 
with  the  mineral  acids,  hydrochloric  and  sul- 
phuric acids.  Their  presence  may  be  shown  by 
using  the  reagents  mentioned  under  nitric  acid. 
These  impurities  may  be  removed  by  distilling 
the  acetic  acid  from  a retort  to  which  a little 
potassium  bisulphate  and  bichromate  have  been 
added. 

Di-Carhoxylic  Acids. — Oxalic  acid,  H2C2O4  or 

COOH. 

I 

COOH. 

This  acid  is  obtained  by  oxidising  sawdust,  by 
fusion  with  caustic  potash.  It  is  also  obtained 
when  many  complex  organic  compounds,  such  as 
sugar,  are  heated  with  strong  nitric  acid.  In 
photographic  work  the  acid  is  employed  in  the 
form  of  its  ferrous  salt,  in  the  well-known  ferrous 
oxalate  developer.  Ferrous  oxalate  itself  is  in- 
soluble in  water,  but  is  readily  soluble  in  a solu- 
tion of  potassium  oxalate,  consequently  the  de- 
veloper is  so  arranged  that  this  potassio-ferrous 
oxalate  is  present.  This  condition  is  obtained  by 
mixing  a solution  of  ferrous  sulphate  with  potas- 
sium oxalate,  and  is  produced  in  accordance  with 
the  following  equation  : — 


ORGANIC  OR  CARBON  COMPOUND: 


141 


FeSO,  + 2K,C,0,  = K,Fe(C,OJ,  + K,SO, 


or 


FeSO, 


COOK 
+ 2 I 

COOK 


COOK 

i 

COOK 


COO. 

+ 1 >Fe  + K,SO, 

coo-^ 


Potassio-fei'rous  oxalate. 


Action  of  Light  on  Ferric  Oxalate. — It  is  inter- 
esting to  notiee  that  a ferric  oxalate  solution, 
which  is  the  compound  produced  after  the  ferrous 
oxalate  developer  has  done  its  work,  is  recon- 
verted to  the  ferrous  state  by  exposure  to  the 
action  of  sunlight. 


Fe,(CA)a  = 2FeC,0,  + 200^ 

Ferric  oxalate.  Ferrous  oxalate. 


Because  oxalic  acid  contains  two  carboxyl  groups, 
it  therefore  forms  two  classes  of  salts.  Thus  : — 


COOH 

1 

COOH 


COOK 

I 

COOH 

Acid  potassium 
oxalate. 


COOH 

I 

COOK. 

Neutral  potassium 
oxalate. 


Oxalic  acid  and  its  salts  are  powerful  reducing 
agents,  and  it  is  owing  to  this  fact  that  they  find 
employment  in  photography.  If  they  are  added 
to  solutions  of  gold,  silver,  or  platinum,  the 
metal  is  precipitated.  Oxalic  acid  crystallises 
with  two  molecules  of  water. 

H yclroxy-tri-C  arhoxylic  Acids.  — Citric  acid. 


CH2(C00H)— CH(OH)COOH— CHXOOH. 

This  acid  forms  three  classes  of  salts,  because  it 
contains  three  carboxyl  groups.  The  three  potas- 
sium salts  are  given  below. 


CH^COOH 

I 

OH-C— COOH 

I 

CH.,COOH 

Citric  acid. 


CH,COOK 

OH-C-COOH 

i 

CH.COOH 

Primary  potassium  citrate. 


142 


PHOTOGKAPHIC  CHEMISTRY. 


CH^COOK 

OH-C-COOK 

CH^COOH 

Secondary  potassium 
^citrate. 


CH^COOK 

I 

OH-C-COOK 

I 

CH2COOK 

Tertiary  potassium 
citrate. 


Sensitisers. — Citric  acid  and  the  citrates,  in 
the  presence  of  the  silver  haloids,  increase  the 
photo-decomposition  of  the  latter,  and  make  them 
more  sensitive  to  the  action  of  light.  Hence  they 
are  often  spoken  of  as  sensitisers. 

Family  of  the  Ketones. — Practically  the  only 
ketone  used  in  photographic  work  is  the  di-methyl 
ketone,  or,  as  it  is  usually  termed,  acetone.  This 
compound  is  obtained  technically  from  crude  wood 
spirit  or  by  the  dry  distillation  of  calcium  acetate. 
On  the  addition  of  sodium  or  potassium  bi-sul- 
phite to  acetone  a white  crystalline  compound  is 
obtained  known  as  acetone  alkaline  bi-sulphite. 

Amiclo  Carboxylic  xicids. — Glycin,  or  glycocoll, 
NH2 — CH2 — COOH.  This  compound  is  acetic 
acid  in  which  one  hydrogen  atom  of  the  methyl 
group  has  been  replaced  by  an  amido  group 
(NH2). 

H NH2 

I I 

CH2COOH  CH2COOH. 

Acetic  acid.  Glycocoll  or  amido 

acetic  acid. 


The  photographic  developer  known  by  the  namt 
of  “ glycin  is  glycocoll  in  which  one  of  the 
amidic  hydrogen  atoms  has  been  replaced  by  the 
complex  group — CfHj  (OH) — present  in  carbolic 
acid  or  phenol.  This  CgH^  (OH)  group  is  cyclic 
in  structure,  consequently  the  developer  is  both 
a cyclic  and  open  chain  compound.  Its  formula 
is  CsHgOgNi. 


143 


CHAPTER  XIII. 

PYROXYLINE,  ALBUMEN,  GELATINE,  ETC. 

Complex  Organic  C om-pounds. — There  are  a large 
number  of  organic  compounds  of  a verj^  complex 
nature  whose  constitution,  up  to  the  present,  has 
not  been  determined.  It  so  happens  that  these 
bodies  are  indispensable  in  photography,  and  the 
art  has  been  brought  to  a great  state  of  perfec- 
tion by  their  employment.  Among  them  may  be 
mentioned  cellulose,  albumen,  and  gelatine. 

Cellulose  (Ci2H2oOio)x. — This  is  the  principal 
constituent  of  the  cell  membranes  of  all  plants. 
It  may  be  obtained  in  a pure  form  by  submitting 
wadding  or  plant  fibre  to  the  action  of  (1)  dilute 
potash,  (2)  dilute  hydrochloric  acid,  washing  with 
water  in  each  case.  It  is  then  treated  with  alcohol 
and  ether.  So  obtained,  it  is  a white  amorphous 
mass.  Swedish  filter  paper,  which  has  been  sub- 
mitted to  these  reagents,  consists  almost  entirely 
of  pure  cellulose.  This  substance  is  practically 
insoluble  in  all  the  usual  solvents,  but  dissolves, 
without  undergoing  any  change,  in  ammoniacal 
copper  solutions.  Acids  reprecipitate  it  as  a 
gelatinous  mass.  It  is  soluble  in  concentrated 
sulphuric  acid,  depositing  a starch-like  compound 
on  the  addition  of  water. 

Cellulose  Nitrates,  or  “ Nitro  ” C elluloses. — 
It  has  already  been  noticed  that  by  replacing  the 
hydrogen  in  nitric  acid  by  a metal  the  nitrates  are 
obtained.  Organic  radicals  or  groups  of  radicals 
can  also  replace  the  hydrogen  of  nitric  acid, 
forming  an  organic  nitrate.  Thus  : — 

C.H.O  -h  HNO3  = C2H3ONO2  + H2O 

Ethyl  nitrate. 


144 


PHOTOGRAPHIC  CHEMISTRY. 


If  cellulose  is  treated  with  nitric  acid  various 
nitrates  are  obtained. 

+ HNO3  = C,3H,3(0N0,)0,  + H,0 

Cellulose  mono-nitrate. 

-1-  2HNO3  = C,,H.,(ONO,)A  + 2H,0 

Cellulose  di-nitrate. 

It  will  be  noticed  that  water  is  produced  in  the 
reaction.  To  remove  this,  and  thus  prevent  it 
from  diluting  the  nitric  acid,  strong  sulphuric 
acid  is  used.  The  resulting  nitrates  exhibit  varj"- 
ing  properties  depending  upon  their  method  of 
formation. 

Gun-G otton,  or  Pyroxyline. — If,  for  example, 
pure  cotton  is  immersed  two  or  three  times  in  a 
cold  mixture  of  one  part  of  nitric  and  three  of 
sulphuric  acid,  and  then  washed  with  water,  it  is 
converted  into  cellulose  hexa-nitrate,  which  is 
known  as  gun-cotton,  or  pyroxyline. 

+ 6HNO3  = C,,H,,(0]SrO,)A  6H,0 

Cellulose  hexa-nitrate, 
or  pyroxyline. 

This  hexa-nitrate  is  insoluble  in  alcohol  and  ether. 

Collodion. — If  cotton  is  exposed  to  the  action 
of  a warm  mixture  of  twenty  parts  of  powdered 
potassium  nitrate  and  thirty  parts  of  concen- 
trated sulphuric  acid,  a mixture  of  tetra  and 
penta  cellulose  nitrates  is  obtained,  which  dis- 
solves in  ether  containing  a little  alcohol.  This 
is  termed  soluble  pyroxyline,  and  the  ether  alcohol 
solution  is  termed  collodion. 

+ 4HNO3  = +4H,o 

Cellulose  tetra-niti  ate. 

5HNO3  = C„H„(0N0,),0,  -F  5H,0 

Cellulose  penta-nitrate. 

On  allowing  collodion  to  evaporate  the  pyroxy- 
line is  left  as  a uniform  transparent  film.  To 
render  it  photographically  sensitive,  the  collo- 
dion is  treated  with  varying  mixtures  of  some 
soluble  iodide  and  bromide,  usually  the  am- 
monium and  cadmium  compounds.  The  coated 


PYROXYLINE,  ALBUMEN,  GELATINE,  ETC.  145 


plate  is  then  dipped  in  a solution  of  silver  nitrate, 
by  which  means  it  is  covered  with  a layer  of  silver 
bromide  and  iodide. 

The  equations  representing  the  changes  are 


NH,1  + AgNOa  = Agl  + NH^NOg 


Ammonium  Silver  Silver  Ammonium 

iodide.  nitrate.  iodide.  nitrate. 


Cd 


Bra  + 2Ag 


Cadmium 

bromide. 


NO3  = 2AgBr  + Cd(N03)a 

Silver  Cadmium 

bromide.  nitrate. 


The  prepared  plate  is  exposed  whilst  still  wet 
with  the  silver  nitrate  solution,  as  this  is  the 
sensitiser.  This  is  a very  important  point  to  be 
noticed,  in  connection  with  the  wet  collodion 
process,  as  the  collodion  by  itself  has  no  halogen 
absorbing  power. 

Celluloid. — The  mono,  di,  and  tri  nitro  com- 
j pounds  of  cellulose  are  dissolved  in  special  sol- 
I vents,  such  as  acetone  and  camphor,  thereby  pro- 
ducing plastic  masses  known  under  the  names  of 
celluloid  and  xylonite,  which  can  be  moulded  and 
cut  into  various  forms. 

I Albumen. — The  molecular  magnitude  of  albu- 

men is  unknown.  According  to  Sabanejeff,  the 
[ number  15,000  was  obtained  as  the  molecular 
[I  weight  of  purified  egg  albumen.  Stohmann  and 
I Langbein  have  given  albumen  the  molecular 
formula  0720111134850248X218.  This  may  or  may  not 
be  the  true  formula,  but  it  is  sufficient  to  show 
I that  albumen  contains  a very  large  number  of 
I atoms  in  the  molecule.  For  photographic  use, 

I purified  egg  albumen  (or  white  of  egg)  is  em- 
I ployed.  This  is  soluble  in  water,  but  if  heated 
! to  a temperature  of  about  70°  C.  becomes  insol  u- 
I ble,  or,  as  it  is  termed,  coagulated.  Many  other 
j substances  also  coagulate  albumen,  such  as  alum, 

: J 


146 


photOgeaphio  chemistey. 


nitric  acid,  methylated  spirit,  and  many  metallic 
salts.  ^ When  albumen  is  treated  with  silver 
nitrate,  an  insoluble  compound  is  precipitated 
containing  silver.  This  compound  is  either  a 
salt,  or  double  compound,  of  the  silver  and  albu- 
men, and  it  is  generally  termed  silver  albuminate. 
Under  the  influence  of  light  it  suffers  decomposi- 
tion, forming  brown-red  reduction  compounds. 
The  sensitive  surface  of  albumen  paper  consists 
of  a mixture  of  silver  chloride,  silver  albuminate, 
together  with  an  excess  of  silver  nitrate. 

Gelatine. — When  bones,  hoofs,  etc.,  are  sub- 
mitted to  the  action  of  superheated  steam,  various 
nitrogenous  substances  are  extracted,  which 
separate  from  their  watery  solution  as  a jelly  on 
cooling.  In  this  form  it  is  termed  “ size.’'  If  it 
is  dried  a hard  mass  results,  forming  the  glue  of 
commerce,  by  the  purification  of  which  the  gela- 
tine is  obtained.  Isinglass  is  a form  of  gelatine 
obtained  from  the  air-bladder  of  the  sturgeon. 
So  far,  no  molecular  formula  has  been  assigned  to 
gelatine,  but  from  what  has  been  ascertained,  it 
appears  to  contain  a great  number  of  atoms  in 
the  molecule.  Tannic  acid  precipitates  gelatine 
from  its  aqueous  solutions  as  gelatine  tannate,  a 
brownish-yellow  sticky  substance.  Gelatine  swells 
considerably  in  water,  but  does  not  dissolve  till 
heated.  On  cooling,  it  separates  as  a gelatinous 
mass.  According  to  Eder,  a good  specimen 
should  produce  a firm  jelly  when  a 4 per  cent, 
solution  is  cooled  down  to  20°  C.  On  boiling  with 
dilute  acids,  or  alkalis,  gelatine  undergoes  de- 
composition, producing  complex  mixtures  of 
amido  fatty  acids.  Aqueous  solutions  of  gelatine 
slowly  decompose  on  standing,  producing  am- 
monia, and  substituted  ammonias. 

Photographic  Emulsions. — As  already  stated, 
solutions  of  gelatine  have  the  property  of  gelatin- 
ising, i.e.  separating  as  a jelly,  on  cooling,  and 
it  is  this  property  which  makes  that  substance  so 


PYROXYLINE,  ALBUMEN,  GELATINE,  ETC.  147 

important  in  preparing  photographic  emulsions. 
A gelatino-haloid  emulsion  consists  of  a solution 
of  gelatine  in  water,  of  such  a degree  of  viscosity 
that  a finely  divided  precipitate  of  silver  haloid 
is  kept  in  a state  of  suspension.  This  is  secured 
in  practice  by  heating  a solution  of  gelatine  with 
silver  nitrate,  potassium  iodide,  and  bromide, 
and  then  allowing  to  set. 

AgNOg  + KBr  = AgBr  + KNO3 
AgN03  + KI  = Agl  + KNO3 

A very  important  point  must  be  noticed  here; 
that  is,  to  have  sufficient  of  the  potassium  haloids 
to  precipitate  all  the  silver.  Unless  this  is  done 
the  silver  nitrate  combines  with  some  of  the  gela- 
tine to  form  a double  insoluble  compound,  which 
undergoes  some  decomposition  during  the  heating. 
When  plates  covered  with  such  an  emulsion  are 
developed,  this  silver  “ gelatino-nitrate  ’’  attacks 
the  developer  and  causes  a general  fog.  Gelatine 
and  collodion  emulsions  have  to  undergo  a pro- 
eess  of  ripening  in  order  to  increase  their  sensi- 
tiveness to  light. 

Hardening  of  Gelatine. — The  various  alums, 
such  as  ordinary  potash  alum  and  chrome  alum, 
and  formaldehyde  or  formalin,  have  the  property 
of  rendering  gelatine  hard,  and  making  it  in- 
soluble in  water.  Hence  the  use  of  these  sub- 
stances for  the  prevention  of  “ frilling.” 
Chromium  compounds  have  also  this  property, 
especially  in  the  presence  of  light,  and  this  is 
taken  advantage  of  in  the  various  bichromate 
printing  processes. 


148 


CHAPTER  XIV. 

BENZENE  AND  THE  ORGANIC  DEVELOPERS. 

Cyclic  or  Aromatic  Compounds. — The  most  im- 
portant cyclic  substances  used  in  photographic 
work  are  the  developers,  pyrogallol,  hydroquinone, 
metol,  amidol,  etc.  Before  these  compounds  are 
considered  it  is  necessary  to  have  some  idea  of  the 
properties  of  a few  simple  cyclic  substances,  the 
nomenclature  used,  etc.  * 

Benzene.  C — This  is  the  parent  hydro- 
carbon from  which  an  immense  number  of  cyclic 
compounds  can  be  derived.  Benzene  is  of  very 
stable  behaviour  towards  chemical  reagents.  The 
halogens  act  in  two  ways  towards  the  hydro- 
carbon : (1)  they  replace  hydrogen  atoms  (substi- 
tuted compounds) ; (2)  they  simply  add  themselves 
on  to  the  benzene  (additive  compounds). 

(1)  + CL  = CeH.Cl  + HCl 

C,H,C1  + CL  = CeH.Cl^  -h  HCI,  etc. 

(2)  C,H,  + Br,  - C.HeBr,  + HBr 

CyH^Br  -1-  Bi\  = CgHeBr^  + HBr,  etc. 

Another  point  to  be  noticed  is  that  benzene  does 
not  polymerise.  If  the  properties  of  benzene  are 
compared  with  those  of  the  fatty  or  open  chain 
compounds  they  are  found  to  differ  in  a very 
marked  manner.  Consequently  the  constitution 
of  benzene  cannot  be  represented  as  related  in  any 
manner  to  the  open  chain  hydrocarbons. 

Distinction  between  Bem.^ne  and  Benzine. — 
Benzene  is  used  directly  in  photography  as  a sol- 
vent. Attention  may  here  be  drawn  to  the  dis- 
tinction between  benzene  and  benzme,  since  both 
these  substances  are  used  as  solvents,  and  appar-  ' 


BENZENE  AND  THE  OKGANIC  DEVELOPERS.  149 


ently  a great  amount  of  confusion  exists  as  to  the 
identity  of  the  two.  Benzene,  the  compound  at 
present  under  consideration,  is  obtained  during 
the  distillation  of  coal  tar.  Benzine  is  a mixture 
of  open  chain  paraffin  hydrocarbons,  a totally 
different  substance.  Indirectly,  benzene  is  used 
in  the  preparation  of  most  of  the  organic  de- 
velopers and  coloured  substances  used  in  colour 
photography. 

Benzene  Derivatives. — By  replacing  one  or 
more  hydrogen  atoms  in  the  benzene  molecule,  by 
other  atoms,  or  group  of  atoms,  various  deriva- 
tives are  obtained.  The  introduction  of  hydroxyl 
groups  produces  the  various  phenolic  bodies  : — 

C,He  C.H^OH  C,H,(OH),  C,H3(OH)3 

Benzene.  Phenol.  ' ^ Hydroquinone.  Pyrogallol. 

These  compounds  are  acid  in  behaviour,  conse- 
quently they  dissolve  in  alkalis.  They  are 
designated  mono,  di,  tri  hydric  phenols  accord- 
ing to  the  number  of  hydroxyl  groups  present. 

Phenol  or  Carbolic  Acid.  C — This  com- 
pound is  obtained  during  the  distillation  of  coal 
tar.  It  is  the  simplest  phenol  and  readily  dis- 
solves in  alkalis.  It  is  employed  as  a preserva- 
tive in  solutions  of  albumen  or  gelatine,  and  for 
mountants.  Nearly  all  phenols  give  definite 
colour  reactions  with  a solution  of  ferric  chloride. 
Ordinary  phenol  gives  a violet  coloration.  Ben- 
zene is  formed  when  phenol  vapour  is  passed  over 
heated  zinc  dust. 

Pyrocatecliol^  Resorcinol.,  Three  com- 

pounds are  known,  having  different  chemical  and 
physical  properties,  yet  all  having  the  molecular 
formula  CcH4(OH)2.  These  three  substances  are 
pyrocatechol,  resorcinol,  and  hydroquinone.  In 
order  to  distinguish  between  three  isomeric  di- 
hydroxy-benzenes they  have  received  special 
names.  Formula  (1)  is  termed  the  ortho ^ (2)  the 
metaj  and  (3)  the  para  derivative. 


150 


PHOTOGRAPHIC  CHEMISTRY. 


OH 

1 

OH 

1 

OH 

1 

A 

A 

A 

HC  C-OH 

1 II 

HC  CH 

HC  CH 

1 II 

HC  CH 

1 II 

HC  .C-OH 

HC  iu 

/ 

•c 

C 

c 

E 

1 

I 

H 

OH 

Ortlio-di-hydroxy- 

Meta-di-liydroxy- 

Para-di-hydroxy- 

benzene. 

benzene. 

benzene. 

Pyrocatechol. 

Resorcinol. 

Hydroquinone. 

All  these  compounds  can  act  as 

photographic 

developers,  but  the  most  important  is  the  para 
derivative,  hydroquinone. 

Hydroquinone,  Para-di-hydroxy-benzene. — This 
compound  was  first  suggested  as  a photographic 
developer  by  Captain  (now  Sir  W.)  Abney  in 
1880.  It  acts  more  slowly  than  pyrogallol,  and 
produces  rather  hard  negatives.  It  is  most  con- 
veniently prepared  by  reducing  a body  known  as 
quinone  (obtained  by  oxidising  aniline,  CeHgNHg) 
with  sulphurous  acid. 


+ Ha 
Quinone. 


O + HaSOa 


= CeH.(OH),  + 

Hydroquinone. 


HaSO, 


Hydroquinone  is  di-morphous  and  dissolves 
readily  in  water.  Its  aqueous  solution  is  coloured 
brown  with  ammonia.  Oxidising  agents  convert 
it  into  quinone. 

Tri-hydroxy  Derivatives  of  Benzene. — These 
compounds  exist  in  three  isomeric  forms,  pro- 
duced, like  the  di-hydric  phenols,  by  the  position 
of  the  hydroxyl  groups  in  the  benzene  molecule. 


BENZENE  AND  THE  ORGANIC  DEVELOPERS.  151 


OH 

ci 


OH 


//  \ ^ N 

HC  C-OH  H-C  C-H 


OH 

I 

C 

//\ 

H-C  C-H 


Hi 


C-OH  H 


u 

C 

OH 


OH  HO-C  C-OH 

V 

H 


Pyrogallol  Hydroxy  Phlorogluciuol. 

or  • hydroquinone. 

Pyrogallic  acid. 

V ^ > 

Isomeric  tri-hydric  phenols. 

Pyrogallol.  G is  the  only 

compound  of  the  three  isomeric  substances  used 
in  photographic  work.  It  melts  at  132°  C.,  and 
is  produced  by  heating  gallic  acid.  The  reaction 
expressing  the  change  is  this  : — 


OH 


OH 


A 

HO-C  CH 
HO-i  ^-COOH 

H 


A 

HO-C  CH 

1 II  + COa 
HO-C.  CH 

H 


Gallic  acid.  Pyrogallol. 

Or  it  may  be  written : 

c„h,(oh)3COoh  z=  c,n,{on),  + co^ 

Gallic  acid.  Pyrogallic  acid. 

Pyrogallol  is  extremely  soluble  in  water,  and  with 
more  difficulty  in  alcohol  and  ether.  Its  alkaline 
solutions  absorb  oxygen  with  great  readiness, 
carbon  di-oxide,  oxalic  and  acetic  acid,  and  vari- 
ous brown  colouring  matters  are  produced  during 
the  oxidation.  A blue  colour  is  imparted  to  pyro- 
gallol solutions  by  ferrous  sulphate,  and  a red  by 
ferric  chloride.  An  iodine  solution  is  turned  a 
purple  red  on  the  addition  of  an  aqueous  or 


162 


PHOTOGRAPHIC  CHEMISTRY. 


alcoholic  solution  of  pyrogallol.  The  latter  is  a 
powerful  reducing  agent,  readily  precipitating 
gold,  silver,  and  mercury  from  their  solutions. 

Amido-Phenolic  Substances. — These  compounds 
are,  like  the  di-  and  tri-hydric  phenols,  used  in 
photography  as  developers.  If  ordinary  phenol 
is  taken  and  treated  with  nitric  acid,  under  the 
proper  conditions,  a mixture  of  ortho  and  para* 
nitro-phenols  is  obtained.  That  is,  a hydrogen 
O O 


93 

Cl 


0—0  ••  ^ 
93  / \ C 0 2: 

C O -2:  k — \ / 

% * 
0-0  X 


' ,53 
^ o 


X / 
0-0^ 


o 

• X 
0=0 


\ 


o-= 


0--0 

93  93 


s o 
o 

\ / 
X 

fO 

o 


O a> 

o 


n 

c 

X 

CM 


o 

2: 

ii: 

CM 


X X 
0=0 

X / \ 

0-0  ox 

^0-0^ 

X X 


93 

o 


o 

CM 


BENZENE  AND  THE  OllGANIC  DEVELOPERS.  153 


atom  in  the  phenol  is  replaced  by  a nitro  group, 
NO2  (see  diagram,  p.  152). 

It  must  be  carefully  noted  that  only  hydrogen 
connected  with  a carbon  atom  of  the  benzene  ring 
is  replaced  by  the  nitro  group ; the  hydroxylic 
hydrogen  remains  intact;  this  is  proved  by  the  in- 
creased acid  properties  of  the  compound  formed. 
If  the  para-nitro-phenol  is  then  separated  and 
treated  with  any  reducing  mixture,  the  nitro 
group  (NO2)  is  converted  into  an  amido  group 
(NH2),  and  para-amido-phenol  results.  Thus  : — 

NO2  + 3H2  = NH2  + 2H2O 

Nitro  group.  Amido  group. 


OH 

1 

OH 

1 

I 

c 

HC 

HC  CH 

1 II  + 3H2 

= 1 11 

HC  CH 

HC  CH 

1 

NOe 

1 

NHj 

Para-nitro- 

Para-amido- 

phenol. 

phenol. 

+ 2H2O 


These  amido-phenols  are  both  basic  and  acid  in 
character.  The  hydroxyl  group  confers  acidic, 
and  the  amido  group  basic  properties  upon  the 
compounds. 

RodinaL — This  developer  is  a strong  solution 
of  the  hydrochloride  of  para  amido-phenol.  The 
hydrochloric  acid  unites  with  the  amido  group. 


.OH 

C.H.^  + HCl 

''NH, 

Para-ainido-plienol. 


.OH 

- C.H  / 

^NK.HCl 

Rodinal,  or  hydrochloride 
of  i>ara-aniido-phenoI. 


Di-ainido-phenols. — By  introducing  two  nitro 
group  into  ordinary  phenol,  or  another  nitro 
group  into  para-nitro-phenol,  di-nitro-phenol  is 
obtained. 


154 


PHOTOGEAPHIO  CHEMISTEY. 


This  compound,  on  treatment  with  reducing 
agents,  is  converted  into  di-amido-phenol.  The 
change  taking  place  may  be  represented  as  fol- 
lows : — 


+ 2H«0 


QH 

H 

HC  CH 

+ 2HNO,  = 1 II 

HC\  CH 

HC,  CH 

^C^ 

H 

k'o. 

or 

T 

C.H.OH 

.2HNO, 

Phenol. 

Di-nitro-f)henol. 

On  reduction: — 

OH 

OH 

+ 2H,0 


NO, 


A 


+ 6H.,  = 


HC  C-NH 


+ 4H^O 


HC  C-H 

HC  CH 

^C^ 

1 

1 

NO, 

NH, 

I . 

T 

+ 6H,  » +4HjO 

\(NO,), 

Nnhj. 

Di-nitro-phenol. 

Di-amido-phenol. 

Amidol. — The 

salts  of  di-amidol-phenol  are 

employed  in  photography  under  the  name  of 
“ amidol,’’  as  developers.  It  is  a powerful  reduc- 
ing agent,  but  is  very  unstable,  and  does  not  keep 
for  any  length  of  time.  It  might  be  noted  here 
that  the  greater  the  number  of  amido  groups  a 
developer  contains,  the  more  unstable  it  becomes. 


BENZENE  AND  THE  OIIGANIC  DEVELOPERS.  165 


Metol. — Another  important  developer  deserves 
consideration  in  connection  with  these  amido 
phenolic  bodies,  namely,  metol.  This  compound  is 
a derivative  of  methyl  phenol,  or,  to  give  it  its 


common  name, 

cresol. 

OH 

1 

OH 

OH 

1 

HC  CH 
1 11  - 
HC  C-CHa 

H 

T 

h 

HC  C-H 
1 li- 

ne C-CHa 

1 

A 

H-C  CH 
1 II 

HC  C-CHj 

V 

1 

NHCH3 

or,  1 

T 

T 

/OH 

-4  C,H3<-CH3  - 

/OH 

CeH3<CH3 

^NHCH, 

Cresol  or  meta-metliyl- 
Iihouol, 

Para-ainido- 

ineta-cresol. 

Metol. 

Para-metliyl- 

amido-meta- 

cresol. 

Motol  is  an  excellent  developer,  and  is  preferred 
by  many  photographers  to  pyrogallol.  . It  is  scld 
as  the  sulphate  of  the  base. 


OH 

2C«H3-CH3 


f H,SO. 


OH 

C.Hy-CH, 


^NHCH,j2 


H.SO, 


Metol  developer. 


N aijhthalene,  Naphthol,  etc. — Another  impor- 
tant aromatic  or  cyclic  compound  is  the  hydro- 
carbon naphthalene.  This  hydrocarbon  consists  of 
two  benzene  rings  joined  together.  Like  benzene 
it  forms  phenolic  bodies  (naphthols)  and  amido- 
phenolic  substances  (amido-naphthols)  similar  in 
properties  to  phenol  and  amido-phenol.  The 
isomeric  derivatives  of  naphthalene  are  very 
numerous,  one  substituent  yielding  two  isomeric 
compounds.  In  this  particular  it  differs  essenti- 
ally from  benzene. 


156 


PHOTOGRAPHIC  CHEMISTRY. 


Eikonogen. — One  important  derivative  of 
naphthalene,  from  a photographic  point  of  view, 
is  the  developer  eikonogen. 

This  was  discovered  by  Professor  R.  Meldola, 
and  placed  on  the  market  as  a photographic  de- 
veloper by  Dr.  Andresen.  Its  preparation  from 
naphthalene  would  be  too  complex  to  consider 
here.  Its  constitutional  formula  is  as  follows  : — ■ 


NH, 

H I 

h/V%-oh 

I II  I or,  C,„H,-OH 
HSO3-C  C CH  \hSO 

■C^ 

H H 


Eikonogen. 


It  will  be  noticed  that  the  developer  contains 
three  groups — an  amido  group  (NH2),  a hydroxyl 
group  (OH),  and  a sulphonic  acid  group  (HSO3), 
united  to  a naphthalene  residue.  The  HSO3  group 
is  very  acid  in  character,  and  readily  forms  salts 
with  alkalis.  The  developer  is  not  very  soluble 
in  water,  and  its  solutions  do  not  keep  very  well. 
It  is  particularly  useful  for  the  development  of 
under-exposed  plates,  owing  to  the  softness  and 
absence  of  chalkiness  characteristic  of  its  results. 
It  would  appear  that  only  those  compounds  which 
contain  the  basic  and  hydroxyl  groups  in  the 
para  ’’  position  are  remarkably  effective  as 
photographic  developers,  thus  showing  how  inti- 
mate is  the  connection  between  photographic  be- 
haviour and  chemical  constitution. 


INDEX 


Absorption,  Halogen,  76 
Acetaldehyde,  Formula  for,  138 
Acetic  Acid,  Formula  for,  139 
Acetone,  142 

, Purifying,  21 

Acids  (See  under  distinctive 
name) 

Affinity,  Chemical,  Defined,  26 
Albumen,  145,  146 
Albumenised  Paper,  84,  85 

, Action  of  Light  on, 

85 

Alcohol,  Ethyl,  137 

, Methyl,  137 

Aldehyde  Compounds,  137,  138 
Alkaline  Development,  72 

Earths,  Metals  of,  123 

Alkalis  or  Bases,  97,  98 
Allotropic  Modifications  of  Sil- 
ver, 126,  127 

Allotropism,  Definition  of,  116 
Alum  Compounds,  132 
Amido-carboxylic  Acids,  142 
Amido-phenolic  Substances,  152, 
153 

Amidol  Developer,  154 
Ammonia,  94-96 

, Blackening  Action  of,  81, 

82 

in  Water,  Testing  for,  47 

Ammonium  Carbonate,  101,  102 

Chloride,  110 

, Formula  for,  38,  125 

Nitrate,  96 

Persulphate,  120 

— Thiocyanate,  120,  121 
Anhydrides,  Acid,  97 
Anions  Defined,  32 
Anode  Defined.  32 
Aqua  Regia,  109 
Aromatic  Compounds,  148 
Atomic  Theory,  28 

Weight,  28 

A urates,  130 
Auric  Acid,  130 
Barium  Compounds,  123 
Bases  or  Alkalis,  97,  98 
Benzene  and  Benzine,  148 
Bonds  Representing  Valency, 
37,  38 

Boring  Corks,  14 
Bromide,  37 

Printing,  85,  86 

Bromides  and  Iodides,  112,  113 
Bromine,  107 
Bunsen  Burners  17.  18 
Calcium  Chloride,  112 

Compounds,  123 

in  Water,  Testing  for,  47 

Cane  Sugar,  Formula  for,  36 
Carbolic  Acid,  149 
Carbon.  Analysis  of,  27 


Carbon  Compounds,  134-142 

Dioxide,  51 

Disulphide,  11 

Carbonates,  100,  101 
Carbonic  Acid,  100 
Cathode,  Definition  of,  32 
Cations  Defined,  32 
Cell,  Voltaic,  Defined,  31 
Celluloid,  145 
Cellulose,  143,  144 

Nitrates,  143,  144 

Chalk,  Analysis  of,  24 
Chemical  Affinity  Defined,  26 

Changes  Due  to  Light,  50, 

57 

Compounds  Defined,  26 

Equations,  39-41 

Laws,  24-34 

Reaction,  42 

— - Reduction,  55 

Theory,  24 

Chemistry,  Aim  of,  9 

Applied  to  Photography, 

11,  12 

and  Physics,  10 

Chlorides*  (See  under  distinc- 
tive name) 

— — , Action  of  Light  on,  106 
used  in  Photography,  109, 

no 

in  Water,  Testing  for,  47 

Cliiorine,  37,  103,  104 

-,  Influence  of  Light  on,  105 

, Symbol  for,  35 

Chromium,  Symbol  for,  35 
Citric  Acid,  Formula  for,  141 
Cobalt,  Symbol  for,  35 
Collodio-chloride  Printing-out 
Paper,  85 
Collodion,  144,  145 
Condenser,  Liebig’s,  20 
Constant  Proportion,  Laws  of, 
26,  28,  29 
Copper,  123 

, Analysis  of,  27 

Chloride,  36 

Mixed  wDh  Sulphur,  11 

Sulphate,  Electrolysis  of, 

32,  33 

•,  Symbol  for,  35 

Cork  Boring,  14 
Corrosive  Sublimate,  125 
Crystallisation,  23 

, Dimorphous,  23 

, Water  of,  43,  45,  46 

Cuprous  Oxide,  39 
Cyanide  Compounds,  133 

of  Potassium  on  Silver. 

Action  of,  127 
Cyanogen,  Formula  for,  38 
Cyclic  Compounds,  148 
Daguerreotype  Process.  68 


168 


INDEX. 


Dehydration  Defined,  22 
Density,  Experiments  on,  73 
Developer,  Ferrous  Oxalate,  70 

Organic,  72 

, Ferrous  Sulphate,  68,  69 

Development,  Acid,  68 

, Alkaline,  72 

, Chemistry  of,  67-79 

in  Daguerreotype  Process, 

68 

, Latent  Image  and,  68 

— — - by  Mercury  Vapour,  68 

-,  Neutral,  75 

-with  Restrainr?'3,  69,  70 

Di-amido-phenol,  15c» 
Di-carboxylic  Acids,  140 
Dimorphous  Crystallisation,  23 
Dissociation  Theory,  31 

■  , Apparatus  for,  19,  20 

Distillation,  18,  21 
Divalent  Elements,  37 
Eikonogen,  156 
Electrodes,  32 
Electrolysis,  31 

of  Copper  Sulphate,  32.  33 

-,  Experiments,  31 

Electrolyte  Defined,  32 
Element  Defined,  24 

, Divalent,  37 

, Molecules  of,  30 

• , Monovalent,  37 

, Radicals,  Compound,  38 

, Univalent,  37 

- — , Valency  of,  Bonds  to  Re- 
present, 37,  38 
Emulsions,  146,  147 

, Ripening  of.  66 

Equations,  Chemical,  35-43 
Ethyl  Alcohol,  137 

, Formula  for,  137 

Evaporation,  17,  18 

, Apparatus  for,  17,  18 

Ferric  Chloride,  110 
Reducer,  79 

■  Oxalate,  141 

Ferricyanide  Reducer,  80 
Ferricyanides,  133 

Ferrous  Oxalate  Developer,  70 

, Action  of  Tiiio- 

sulpliate  in,  71 

, Meldola  and,  72 

, Reactions  of,  70, 

71 

Intensifier,  82,  83 

Sulphate  Developer,  68,  69 

Filtration,  14,  15,  16 

Fixing  by  Thiosulphates,  118, 

119 

Flasks,  Glass,  48 
Fluorine,  107 
Formaldehyde.  137,  138 
Formic  Acid,  Test  for,  140 
Funnel,  Thistle,  48 
Furfurol,  Test  for,  140 
Gelatine.  146 


Gelatine,  Hardening,  147 
Gelatino-chloride  Printing-out 
Papers,  85 
Glass  Flasks,  48 
— — Jet,  Making,  16 

Tubing,  Bending,  14 

Glycin  Developer,  142 
Gold  Chloride,  111 

, Decomposition  of,  18 

, Formula  for,  36 

Salts,  129,  130 

, Symbol  for,  35 

Toning,  87 

Gramme  Defined,  44 
Green  Fog,  Removal  of,  80,  81 
Gun-cotton,  144 
Halogen  Absorption,  76 

, Explanation  of  Term,  103 

Halogens.  Hydrogen  Com- 
pounds of,  108 
Haloids,  Mercury,  124 

, Silver,  128 

Heat  Unit  of  Water,  44,  45 
Hydriodic  Acid,  37,  112 
Hydrobromic  Acid,  37,  112 
Hydrochloric  Acid,  37,  38,  41,  109 
Hydrofluoric  Acid,  113 
Hydrogen,  Atomic  Weight,  28 

Chloride,  108 

Compounds  of  Halogens, 

108 

— — Experiments  Avith,  53,  54 

, Nascent,  54 

— —,  Preparation  of,  52,  53 

, Sulphuretted,  39 

Hydroquinone  Developer,  149, 
150 

Hydroxyl,  Formula  for,  38,  141 
Hydroxy-tri-Carboxylic  Acids, 
141 

Hypo  from  Sulphur  Dioxide, 
117 

Indestructibility  of  Matter,  12, 
13 

Intensification,  81 

with  Ferrous  Oxalate,  82. 

83 

Lead,  83 

Sodium  Sulphite,  82 

Uranium,  83,  84 

Iodides  and  Bromides,  112,  113 
Iodine,  37,  107 

, Experiment  with,  22 

Ionic  Theory,  31 
Ions  Defined,  32 

, Reactions  of,  33,  34 

Iron,  Oxide  of,  52 

, , Experiment 

with,  41 

Presence  of,  109 

Rust,  10,  11 

Isomeric  Compounds,  136 
Ketones,  142 
Latent  Image,  58 
, Destruction  of,  75,  76 


INDEX. 


169 


Latent  Image  and  Development, 
68 

, Eder  on,  56,  57 

, Experiments  on,  65 

, Moser’s  Experiments 

on,  65 

, Relapse  of,  64,  65,  75, 

76 

, Theories  concerning, 

56-67 

Law  of  Constant  Proportion, 
26,  28,  29 

Multiple  Proportions, 

26,  27,  28,  29 
Laws,  Chemical,  24-34 
Lead  Intensifter,  83 
Liebig’s  Condenser.  20 
Light|Action,  Reversal  by,  77, 78 
Litmus,  Properties  of,  52 
Magnesium  Metals,  124 

in  Water,  Testing  for,  47 

Manganese  Dioxide.  49 
Matter,  Indestructibility  of,  12, 

Mercuric  Chloride,  125 

, Formula  for,  125 

, Reiss’s  Experiment 

with,  78,  79 

Mercurous  Oxide,  Formula  for. 
39 

Mercury,  124,  125 

— Haloids,  124 

— Vapour,  Development  by, 
68 


Metallic  Chlorides,  Action  oi 
Light  on,  106 
Metals.  Alkali,  122,  123 

, Magnesium,  124 

, Properties  of,  24,  25 

Methyl  Alcohol,  137 
Methylated  Spirit,  21,  22 
Metol  Developer,  155 
Molecular  Disturbance,  76,  77 

Formulae,  36,  37 

for  Organic  Com- 
pounds, 135 

Strain  Theory,  63 

Weight,  30.  31 

Molecule  of  Compounds  30 

Defined,  30,  31 

Molecules  of  Elements,  30 

Silver  Nitrate,  30 

Water,  30 

Mono-carboxylic  Acids,  139 
Monovalent  Elements,  37 
Multiple  Proportion,  26  29 
Naphthalene,  155 
Nascent  Hydrogen.  554 
Neutral  Development,  75 
Nitrates,  Tests  for,  94 
Nitric  Acid,  91-94 
on  Silver,  Action  of. 


, Test  for,  94 

Oxide,  90 


Nitrogen,  Analysis  of.  27 

Compounds,  89-102 

, Obtaining,  89 

, Oxides  of,  89,  90 

Nitrous  Oxide.  90,  91 
Non-metals,  Properties  of,  24, 
25 

Open  Chain  Compounds,  135, 
136 

Organic  Acids,  Family  of,  138, 
139 

Compounds,  134-142 

, Molecular  Formulae 

for,  135 

Developers,  72 

, Explanation  of  Term,  134 

Oxalic  Acid,  38,  140 
Oxidation,  51 
Oxides,  51 

of  Nitrogen,  89,  90 

Oxy-chloride  Theory,  62 
Oxygen,  Experiments  with,  50, 
51 

, Preparing,  49,  50 

Paper,  Aibumenised,  84,  85 

. , Action  of  Light  on, 

85 

Persulphates,  119,  120 
Phenol  or  Carbolic  Acid,  149 
Phosphorus,  Effect  of  Light 
upon,  56 

Photo  Salts  of  Carey  Lea,  61,  62 
Photo-chemical  Extinction,  57, 
58 

Photo-chemistry  of  Silver  Com- 
pounds, 58 
Photo-oxidation,  52 
Photo-reduction,  55 
Physics  and  Chemistry,  10 
Platinic  Chloride,  111,  112,  131 

. Molecular  Formula 

for,  36 

Platinotype,  131,  132 
Platinous  Chloride,  131 
Platinum,  130 

, Experiment  with,  31 

and  Lead  Toning,  87,  88 

, Printing  in.  86 

Polymerism  of  Compounds,  138 
Potassium  Chlorate,  49 

Chloroplatinite,  131 

Cyanide,  Formula  for,  133 

— on  Silver,  Action  of, 

Ferrocyanide,  133 

Iodide  on  Silver,  Action 
of,  127 

Printing.  Bromide,  85,  86 

in  Platinum,  86 

Proportion,  Constant,  28,  29 

, Law  of,  26 

, Multiple,  Laws  of,  28,  29 

Pyrocatechol,  Formula  for  149 

Pyrogallol,  151,  152 
. Moderating,  72 


160 


INDEX. 


Pyrolusite,  49 
Pyroxyline,  144 
Radicals,  Compound,  38 
Rain  Water,  46 

Reaction,  Chemical,  Compound 
Produced  during,  42 
Reactions.  Reversible,  41,  42 
Reagent,  Nessler’s,  47 
Reduced  Silver,  72,  73 
Reducer,  Ferric  Chloride,  79 

. Ferricyanide,  80 

Reduction,  Ammonium  Persul- 
phate used  in,  120 

, Chemical,  55 

, Chemistry  of,  79-81 

Resorcinol,  Formula  for,  149 
Restrainers,  Development  with, 
69,  70 
Retort,  48 

Reversal,  Explanation  of,  78 

by  Light  Action,  77,  78 

Reversible  Reactions,  41,  42 
Ripening  of  Emulsions,  66 
Rodinal,  153 
Rust,  Iron,  10,  11 
Salts,  98 

, Acid,  98,  99 

- — , Analysis  of,  26 

, Carey  Lea’s  Photo,  61,  62 

, Gold,  129,  130 

, Preparation  of,  98,  99 

Selenium,  Effect  of  Light  upon, 
56 

Sensitisers,  67,  142 
Silver,  126 

, Action  of,  on  Mercuric 

Chloride,  125 

— . Nitric  Acid  on, 

127 

— Potassium  Cyan- 

ide on,  127 

— , Iodide  on, 

127 

, Allotropic  Modifications 

of,  126,  127 

Bromide,  128 

Chloride,  Action  of  Light 

on,  9 

, Formula  for,  36,  128 

Compounds,  Photo-chemis- 
try of.  58 

, Electrolytic  Action  of,  75 

Haloids,  128 

Image,  Growth  of,  74,  75 

Iodide,  128,  129 

Nitrate,  30,  36 

, Action  of  Hydro- 
chloric Acid  upon,  41 

, Molecules  of,  30 

, Reduced,  Origin  of,  72,  73 

Soda,  Washing,  43 

, , Molecular  Formula 

of  43 


Sodium  Sulphite,  Action  of  Air 
on,  9 

, Intensification  with, 

82 

Solarisation,  77,  78 
Specific  Gravity,  45 
Spirit,  Wood,  21 
Strontium  Compounds,  123 
Sublimate,  Corrosive,  125 
Sublimation,  22 
Sub-salts  Theory,  59-61 
Sulphates  in  Water,  Testing 
for,  47 

Sulphites,  Reducing  Action  of, 
117 

Sulphocyanide,  120,  121 
Sulphur  and  its  Compounds, 
114-121 

, Copper  mixed  with,  11 

Dioxide,  51,  116,  117 

, Formula  for,  39 

, Hypo  from,  117 

Sulphuretted  Hydrogen,  3C 
Sulphuric  Acid,  109 

, Formula  for,  38,  39 

Symbols,  Meaning  of,  35-43 
Thiocarbamide,  121 
Thiocyanate,  Ammonium,  120, 
121 

Thiosulphates,  117,  118 

, Abney  on  Use  of,  71 

in  Developer,  71 

, Fixing  by,  118,  119 

Removal,  119 

Thio-urea,  121 
Thistle  Funnel,  48 
Toning,  Chemistry  of,  86-88 
Univalent  Elements,  37 
Uranium,  130 

Intensifier,  83,  84 

Toning,  88 

Valency,  Bonds  Representing, 
37,  38 

of  Elements  Defined,  36,  37 

Vapour,  Mercu'ry,  Development 
by,  68 

Voltaic  Cell  Defined,  31 
Wash  Bottle.  15,  16 
Washing  Soda,  43 

, Molecular  Formula 

of,  43 

Water  Bath,  18 

of  Crystallisation,  43,  45. 

46 

, Distillation  of,  20,  21 

, Examination  of,  47 

, Hard,  46 

Heat  Unit  of,  44,  45 

, Impurities  in,  45-47 

— — , Molecules  of,  30 

-,  Properties  of,  44-47 

Wood  Spirit,  21 
Zinc  Electrolysis,  31 


Printed  by  Cassell  and  Company,  Ltd,,  Ludgate  Hill,  London,  E 0. 
10,1113 


HANDICRAFT  SFRIFS  {continued). 


Electro-  Plating.  With  Numerous  Engravings  and  Diagrams. 

Contents. — introduction.  Tanks,  Vats,  and  other  Apparatus.  Batteries^ 
Dynamos,  and  Electrical  Accessories.  Appliances  for  Preparing  and  i inishing 
Work.  Silver-Plating,  Copper-Plating.  Gold-Plating.  Nickel-Plating  and 
Cycle- Plating.  Einishing  Electro-Plated  Goods.  Electro-Plating  with  Vari -us 
Metals  and  Alloys.  Index. 

Clay  Modelling  and  Piaster  Casting.  With  153  Engravings  and 
Diagrams. 

Violins  and  Other  Stringed  Instruments.  With  about  180 

Illustrations. 

Materials  and  Tools  for  Violin  Making,  Violin  Moulds.  Violin 
Making.  Varnishing  and  Finishing  Violins.  Double  Bass  and  a Violoncello. 
Japanese  One-string  Violin.  Mandolin  Making.  Guitar  Making.  Banjo 
Making.  Zither  Making.  Dulcimer  Making.  Index. 

Glass  Writing,  Embossing,  and  Fascia  Work,  (including 
the  Making  and  Fixing  of  Wood  Letters  and  Illuminated  Signs.)  W ith 
129  Illustrations. 

Contents. — Plain  Lettering  and  Simple  Tablets.  Gold  Lettering.  Blocked 
Letters.  Stencil  Cutting.  Gold  Etching.  Embossing.  French  or  d'reble 
Embossing.  Incised  Fascias,  Stall-plates,  and  Grained  Background.  Letters 
in  Perspective  ; Spacing  Letters.  Arrangement  of  Wording  and  Colors.  Wood 
Letters.  Illuminated  Signs.  Temporary  Signs  for  Windows.  Imitation 
Inlaid  Signs.  Imitation  Mosaic  Signs.  Specimen  Alphabets.  Index. 

Photographic  Chemistry,  With  31  Engravings  and  Diagrams. 

Photographic  Studios  and  Dark  Rooms.  With  180  Illus- 
trations. 

Contents. — Planning  Studios.  Building  Studios.  Portable  and  Temporary 
Studios.  Studios  Improvised  from  Greenhouses,  Dwelling  Rooms,  etc. 
Lighting  of  Studios.  Backgrounds.  Scenic  Accessories.  Dark-Rooms.  Portable 
Dark-Rooms.  Dark-Room  Fittings.  Portable  Dark  Tent.  Index. 

Motor  Bicycle  Building.  With  137  Illustrations  and  Diagrams. 

Contents. — Frame  for  Motor  Bicycle.  Patterns  for  Frame  Castings.  Build- 
ing Frame  from  Castings.  Making  3^  H.  P.  Petrol  Motor.  Spray  Carburettor 
for  si  H.  P.  Motor.  Ignition  Coils  for  Motor  Cycles.  Light-weight  Petrol 
Motor  for  Attachment  to  Roadster  Bicycle.  Spray  Carburettor  for  Light- 
weight Motor.  Index. 

Rustic  Carpentry.  With  172  Illustrations. 

Contents. — Light  Rustic  Work,  Flower  Stands,  Vases,  etc.  Tables,  Chairs 
and  Seats.  Gates  and  Fences.  Rosery  Work,  Porch,  Swing  Canopy  Aviary, 
Footbridges  Verandahs.  Tool  Houses,  Garden  Shelters,  etc.  Summer  Houses, 
Dovecot.  Index. 

Pumps  and  Rams!  Their  Action  and  Construction. 

With  1 71  Illustrations. 

Contents. — Suction  Pumps  and  Lift  Pumps.  Making  Simple  Suction  Pumps, 
Pump  Cup  Leathers,  Pump  Valves,  Ram  or  Plunger  Pumps.  Making  Bucket 
and  Plunger  Pump.  Construction  of  Plumbers’  Force  Pump,  Wooden  Pumps, 
Small  Pumps  for  Special  Purposes,  Centrifugal  Pumps,  Air  Lift,  Mammoth, 
and  Pulsometer  Pumps,  Hydraulic  Rams.  Index. 

Domestic  Jobbing.  With  107  Illustrations. 

Contents. — Cutlery  Grinding,  Sharpening  and  Repairing.  Simple  Soldering 
and  Brazing.  China  Riveting  and  Repairing.  Chair  Caning,  Furniture  Re- 
pairing, Glazing  Windows,  Umbrella  Making  and  Repairing.  Index. 
Tinplate  Work.  With  280  Illustrations  and  Diagrams. 

Contents. — Tinmen's  Tools,  Appliances  and  Materials.  Elementary  Ex- 
amples in  Tinplate,  Hollowing  Tinplate.  Simple  Round  Articles  in  Tinplate. 
Saucepan  Making.  Square  and  Oval  Kettle  Making.  Oil  Cooking  Stove. 
Set  of  Workshop  Oil  Cans.  Fancy  Paste  Cutters.  Lamps  and  Lanterns. 
Index. 

Other  Volumes  in  Preparation, 

DAVID  McKAY,  Publisher,  604-608  South  Washington  Square,  Philadelphia. 


TECHNICAL  INSTRUCTION. 

Important  New  Series  of  Practical  Volumes.  Edited  by  PAUL 
N.  HASLUCK.  With  numerous  Illustrations  in  the  Text. 
Each  book  contains  about  i6o  pages,  crown  8vo.  Cloth, 
^l.oo  each,  postpaid. 

Practical  Draughtsmen’s  Work.  With 226 illustrations. 

Contents. — Drawing  Boards.  Paper  and  Mounting.  Draughtsmen’s  Instru- 
ments, ^ Drawing  Straight  Lines.  Drawing  Circular  Lines.  Elliptical  Curves. 
Projection.  Back  Lining  Drawings.  Scale  Drawings  and  Maps.  Colouring 
Drawings.  Making  a Drawing.  Index. 

Practical  Gasfitting.  With  120 illustrations. 

Contents. — How  Coal  Gas  is  Made.  Coal  Gas  from  the  Retort  to  the  Gas 
Holder.  Gas  Supply  from  Gas  Holder  to  Meter.  Laying  the  Gas  Pipe  in  the 
House.  Gas  Meters.  Gas  Burners.  Incandescent  Lights.  Gas  Fittings  in 
Workshops  and  Theatres.  Gas  Fittings  for  Festival  Illuminations.  Gas  Fires 
and  Cooking  Stoves.  Index. 

Practical  Staircase  Joinery.  With  215  illustrations. 

Contents. — Introduction  : Explanation  of  Terms.  Simple  Form  of  Staircase 
— Housed  String  Stair  : Measuring,  Planning,  and  Setting  Out.  Two-flight 
Staircase.  Staircase  with  Winders  at  Bottom.  Staircase  with  Winders  at  Top 
and  Bottom.  Staircase  with  Half-space  of  Winders.  Staircase  over  an  Oblique 
Plan.  Staircase  with  Open  or  Cut  Strings.  Cut  String  Staircase  with  Brackets. 
Open  String  Staircase  with  Bull-nose  Step.  Geometrical  Staircases.  Winding 
Staircases.  Ships’  Staircases.  Index. 

Practical  Metal  Plate  Work.  With  247  Illustrations. 

Contents. — Materials  used  in  Metal  Plate  Work.  Geometrical  Construction 
of  Plane  Figures.  Geometrical  Construction  and  Development  of  Solid 
Figures.  Tools  and  Appliances  used  in  Metal  Plate  Work.  Soldering  and 
Brazing.  Tinning.  Re-tinning  and  Galvanising.  Examples  of  Practical 
Metal  Plate  Work.  Examples  of  Practical  Pattern  Drawing.  Index. 

Practical  Graanirig  and  Marbling.  With  79  Illustrations. 

Contents. — Graining:  Introduction,  Tools,  and  Mechanical  Aids.  Graining 
Grounds  and  Graining  Colors.  Oak  Graining  in  Oil.  Oak  Graining  in  Spirit 
and  Water  Colours.  Pollard  Oak  and  Knotted  Oak  Graining.  Maple  Graining 
Mahogany  and  Pitch-pine  Graining.  Walnut  Graining.  Fancy  Wood  Grain- 
ing. Furniture  Graining  Imitating  Woods  by  Staining.  Imitating  Inlaid 
Woods.  Marbling:  Introduction,  Tools,  and  Materials.  Imitating  Varieties 
of  Marble.  Index. 

Painters*  Oils  Colors  and  Varnishes.  With  Numerous 
Illustrations. 

Contents. — Painters'  Oils.  Color  and  Pigments.  White  Pigments.  Blue 
Pigments.  Chrome  Pigments.  Lake  Pigments.  Green  Pigments.  Red  Pig- 
ments. Brown  and  Black  Pigments.  Yellow  and  Orange  Pigments.  Bronze 
Colors.  Driers.  Paint  Grinding  and  Mixing.  Gums,  Oils,  and  Solvents  for 
Varnishes.  Varnish  Manufacture.  Index. 

Practical  Plumbers*  Work.  With  298  Illustrations. 

Contents. — Materials  and  Tools  Used.  Solder  and  How  to  Make  It.  Sheet 
Lead  Working.  Pipe  Bending.  Pipe  Jointing.  Lead  Burning.  Lead-Work 
on  Roofs.  Index. 

Practical  Pattern  Making.  With  295  Illustrations. 

Contents. — Foundry  Patterns  and  Foundry  Practice.  Jointing-up  Patterns. 
Finishing  Patterns.  Circular  Patterns.  Making  Core  Boxes.  Boring  Holes 
in  Castings.  Patterns  and  Moulds  for  Iron  Columns.  Steam-Engine  Cylinder 
Patterns  and  Core  Boxes.  Worm  Wheel  Pattern.  Lathe  Bed  Patterns. 
Head  Stock  and  Poppet  Patterns.  Slide-rest  Patterns.  Valve  Patterns  and 
Core  Boxes.  Index. 

Practical  Handrailing.  With  144  Illustrations. 

Principles  of  Handrailing.  Definition  of  Terms.  Geometrical 
Drawing.  Simple  Handrails.  Wreathed  Handrails  on  the  Cylindrical  System. 
The  Uses  of  Models,  Obtaining  Tangents  and  Bevels.  Face  Moulds:  their 
Construction  and  Use.  Twisting  the  Wreath.  Completing  the  Handrail. 
Orthogonal  or  Right-angle  System  of  Setting  Wreathed  Handrails.  Handrails 
for  Stone  Stairs.  Setting  out  Scrolls  for  Handrails.  Setting  out  Moulded 
Caps.  Intersecting  Handrails  without  Basements.  Index. 


TECHNICAL  INSTRUCTION  {continued). 


Practical  Brickwork.  With  368  Illustrations. 

Contents. — Englisn  and  Flemish  Bonds.  Garden  and  Boundary  Walls. 
Bonds  for  Square  Angles.  Excavations,  Foundations,  and  Footings.  Junctions 
of  Cross  Walls.  Reveals,  Piers.  Angles  and  other  Bonds.  Jointing  and 
Pointing.  Damp-proof  Courses  and  Construction.  Hollow  or  Cavity  Walls. 
Chimneys  and  Fireplaces.  Gauged  Work  and  Arches.  Niches  and  Domes. 
Oriel  Windows. 

Practical  Painters*  Work.  With  Numerous  illustrations. 

Contents. — Objects,  Principles  and  Processes  of  Painting.  Painters’  Tools 
and  Appliances.  Materials  used  by  Painters.  Preparing  Surfaces  for  Paint- 
ing, Painting  Woodwork,  Painting  Ironwork,  Painting  Stucco  or  Plaster; 
Distempering  and  Whitewashing  Color  Combination.  House  Painting.  Varnish 
and  Varnishing.  Stains  and  Staining.  Estimating  and  Measuring  Painters' 
Work.  Index. 

Textile  Fabrics  and  Their  Preparation  for  Dyeing. 

With  Numerous  Illustrations. 

Contents. — Cotton.  Flax,  Jute,  and  China  Grass.  Wool.  Silk.  Cotton 
BleacMng.  Linen  Bleaching.  Mercerising.  Wool_  Scouring  and  Bleaching, 
Scouring  and  Bleaching  Silk.  Water.  About  Dyeing.  Index. 

Coloring  Matters  for  Dyeing  Textiles.  With  Numerous 
Illustrations  • 

Contents. — Indigo  Coloring  Matters.  Logwood  Coloring  Matters.  Natural 
Red  and  Yellow  Coloring  Matters.  Aniline  Coloring  Matters.  Azo  Coloring 
Matters.  Anthracene  Coloring  Matters.  Chrome  Yellow,  Iron  Buff,  Man- 
ganese Brown,  Prussian  Blue,  Method  of  Devising  Experiments  in  Dyeing. 
Estimation  of  the  Value  of  Coloring  Matters.  Index. 

Sanitary  Construction  in  Building.  With  13 1 Illustrations. 

Contc7its. — Introductory.  Soils,  Subsoils,  and  Sites.  Materials  of  Construc- 
tion. Footings,  Foundations,  and  Damp-proof  Courses.  Stability  of  Walls. 
Roofs.  Floors,  Hearths,  and  Staircases.  Air  Space  and  Ventilation.  A 
Typical  Dwelling.  Index. 

Iron'.  Its  Sources,  Properties,  and  Manufacture.  With 

Numerous  Illustrations. 

Contents. — Introductory,  Terms  Explained.  Refractory  Materials,  Crucibles, 
etc.  Ores  of  Iron.  Metallurgical  Chemistry  of  Iron.  Cast  Iron  or  Pig  Iron. 
Preparation  of  the  Ores.  Changes  in  the  Blast  Furnace.  Blast  Furnace. 
Air  Supply.  Blowing  Engines  Working  the  Blast  Furnace.  By-products. 
Malleable  or  Wrought  Iron.  Production  of  Malleable  Iron.  Preparation  of 
Malleable  Iron  in  Open  Hearths.  Puddling.  Refining  Pig  Iron  and  Dry 
Puddling.  Forge  Machinery.  Iron-rolling  Mill.  Index. 

Steel  I Its  Varieties,  Properties,  and  Manufacture. 

With  132  Engravings  and  Diagrams.  By  William  Henry  Greenwood. 
Revised  and  Rewritten  by  A.  Humboldt  Sexton. 

Contents. — Steel:  Its  Properties  and  Manufacture.  The  Bessemer  Process. 
The  Basic  Bessemer  Process.  Modifications  of  the  Bessemer  Process.  Gas 
Producers  and  the  Siemens  Furnace.  The  Siemens  or  Open-hearth  Steel 
Process.  The  Basic  Open-hearth  Process.  Modifications  of  the  Open-hearth 
Process.  Steel  Works  Appliances.  The  Cementation  and  Monor  Steel  Pro- 
cesses. Casting  Steel.  Forging  and  Rolling  Steel.  Microscopic  Structure  of 
Steel.  Heat  Treatment  of  Steel.  Theory  of  Steel.  Testing  Steel.  Specifica- 
tions of  Steel  for  Various  Purposes.  Alloy  Steels.  Index. 

Other  New  Volumes  in  Preparation. 

DAVID  McKAY,  Publisher,  604-608  South  Washington  Square,  Philadelphia. 


GETTY  RESEARCH  INSTITUTE 


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